LVIV (REUTERS) - The British Ministry of Defence said on Sunday (March 13) that Russian naval forces had established a distant blockade of Ukraine's Black Sea coast, isolating Ukraine from international maritime trade.

The Black Sea is also an important trade and transportation artery for Russia.

Russian naval forces are continuing to carry out missile strikes against targets in Ukraine, the ministry said in its intelligence update posted on Twitter.

Russia could conduct further amphibious landing operations in the coming weeks, similar to the one conducted in the Sea of Azov, the update added.

Meanwhile, United States Secretary of State Antony Blinken on Sunday condemned a Russian attack on a large Ukrainian base near the border with North Atlantic Treaty Organisation member Poland, which killed 35 people and wounded 134, according to a local official.

"We condemn the Russian Federation's missile attack on the International Centre for Peacekeeping and Security in Yavoriv, close to Ukraine's border with Poland," Mr Blinken wrote on Twitter. "The brutality must stop."

Russia's Defence Ministry said the air strike had destroyed a large amount of weapons supplied by foreign nations that were being stored at the sprawling training facility, and that it had killed "up to 180 foreign mercenaries".

The attack on the base, just 25km from the Polish border and which has previously hosted Nato military instructors, brought the conflict to the doorstep of the Western defence alliance.

Russia had warned on Saturday that convoys of Western arms shipments to Ukraine could be considered legitimate targets.

Britain said the incident marked a "significant escalation".

White House national security adviser Jake Sullivan, speaking on a CBS programme, said any attack on Nato territory would trigger a full response by the alliance.

Regional governor Maksym Kozytskyy said Russian planes fired around 30 rockets at the Yavoriv facility and that some were intercepted. At least 35 people were killed and 134 wounded, he said.

Russian Defence Ministry spokesman Igor Konashenkov said Russia had used high-precision, long-range weapons to strike Yavoriv and a separate facility in the village of Starichi.

"As a result of the strike, up to 180 foreign mercenaries and a large amount of foreign weapons were destroyed," he said.

The 360 sq m facility is one of Ukraine's biggest and is the largest in the western part of the country, which has so far been spared the worst of the fighting.

Ukraine, whose aspirations to join Nato are a major irritant to Russian President Vladimir Putin, held most of its drills with Western countries at the base before the invasion. The last major exercises were in September.

In the weeks before Russia's Feb 24 invasion, the Ukrainian military trained there but according to Ukrainian media, all foreign instructors left in mid-February, leaving behind equipment.

"The dining room and dormitory were destroyed. So were the barracks," said Colonel Leonid Benzalo, an officer in the Ukrainian medical reserve who was thrown across the room by one of the blasts.

"The most important thing is we're still alive," he told Reuters after treating the wounded there.

While Western nations have sought to isolate Mr Putin by imposing harsh economic sanctions and have been supplying Ukraine with weapons, the US and its allies are concerned about Nato being drawn into the conflict.

"There are no Nato personnel in Ukraine," the Nato official said, when asked if anyone from the alliance was at the base.

Heavy fighting was reported on multiple fronts.

Air raid sirens wailed across the capital Kyiv and the authorities said they were stockpiling two weeks worth of food items for the two million people who have not yet fled from Russian forces attempting to encircle the city.

Ukraine reported renewed air strikes on an airport in the west and heavy shelling on Chernihiv north-east of the capital.

Interior Ministry official Vadym Denyenko said Ukrainian forces were counter-attacking in the eastern Kharkiv region and around the southern town of Mykolayiv. Reuters was not able to verify those statements.

An American journalist was shot and killed by Russian forces in the town of Irpin, north-west of Kyiv, and another journalist was wounded, the regional police chief said.

Despite the violence, both sides gave their most upbeat assessment yet of prospects for progress at talks held periodically.

"Russia is already beginning to talk constructively," Ukrainian negotiator Mykhailo Podolyak said in a video online.

"I think that we will achieve some results literally in a matter of days."

A Russian delegate to talks, Mr Leonid Slutsky, was quoted by RIA news agency as saying they had made significant progress and it was possible the delegations could soon reach draft agreements.

Neither side said what these would cover. Three rounds of talks between the two sides in Belarus, most recently last Monday, had focused mainly on humanitarian issues.

But there were contradictory statements on the timing of new discussions.

Ukrainian presidential adviser Oleksiy Arestovych told national television "talks are continuing right now".

Kremlin spokesman Dmitry Peskov denied this, saying talks were planned for Monday via video link.

Seeking to up the ante on the increasingly sanctions-hit Russian economy, Ukrainian President Volodymyr Zelenskiy, in a Twitter post, called on US software firms Microsoft and Oracle and German business software group SAP to halt support services in Russia.

Russia's invasion has sent more than 2.5 million people fleeing across Ukraine's borders and trapped hundreds of thousands in besieged cities.

"It is terrifying how violent and inhuman it is," a refugee from Kyiv told Reuters after crossing into Romania.

Ukraine's human rights monitor said Russia used phosphorous bombs in an overnight attack on the town of Popasna in the eastern Luhansk region, calling it a "war crime".

She shared a photograph purporting to show the alleged attack. Reuters could not immediately verify any of the reports.

Phosphorus munitions can be used legally in war to provide light, create smokescreens or burn buildings. But its use in populated areas has been a persistent source of controversy.

In eastern Ukraine, Russian troops were trying to surround Ukrainian forces as they advance from the port of Mariupol in the south and Kharkiv in the north, the British Defence Ministry said.

The city council in Mariupol said in a statement that 2,187 residents had been killed since the start of the invasion.

Reuters was not able to verify that toll.

Kharkiv, Ukraine's second largest city, has suffered some of the heaviest bombardment.

Videos from one resident, Mr Teimur Aliev, showed bombed buildings lining streets, burnt-out cars riddled with shrapnel holes and debris strewn around.

"We will stitch up the wounds and the pain of our country and our city," said Mr Aliev, a 23-year-old musician. "We're not going anywhere."

In Chernihiv, north-east of Kyiv, firefighters rescued residents from a burning building after heavy shelling, video from emergency services - and verified by Reuters - showed.

Moscow denies targeting civilians. It blames Ukraine for failed attempts to evacuate civilians from encircled cities, an accusation Ukraine and its Western allies strongly reject.

Ukrainian Deputy Prime Minister Iryna Vereshchuk said on national television that more than 140,000 people had been evacuated from conflict zones, but that a humanitarian convoy had been unable to reach Mariupol due to shelling.

The Kremlin describes its actions as a "special operation" to demilitarise and "de-Nazify" Ukraine.

Ukraine and Western allies call this a baseless pretext for a war of choice.