KIEV (AFP) - Thousands took to the streets for rival pro- and anti-Kremlin rallies across Ukraine on Sunday as the West and Moscow dug in their heels over a deepening crisis in Crimea.

The protests come after a new marathon round of phone calls by United States President Barack Obama seeking to defuse the Cold War-style standoff in the ex-Soviet state.

Tensions in Ukraine's Black Sea peninsula of Crimea, which has come under de facto control by Russian forces, have resulted in a new face-off between Russia and the West, with each side imposing sanctions or warning of further prohibitive measures to force the other to back down.

Foreign observers have failed to get into Crimea, a majority Russophone region of Ukraine, to get a first-hand look at the situation on the ground and were forced to turn back on Saturday after pro-Kremlin gunmen fired warning shots.

Ukrainian border guards said Saturday that about 60 Russian military lorries had entered the rugged peninsula of two million in the last 24 hours by land and sea, a sign that Moscow was far from pulling back.

They also said "Russian extremists" had attacked a Crimea radar post, in the latest move by pro-Russian forces who have surrounded Ukrainian military bases and taken at least one missile defence unit.

"Armed attackers in military uniform and civil clothes broke the door and entered the building. Radio technic units and telecommunication cables were dissembled and destroyed," the border guards said in a statement in English on their website.

Sunday marks the 200th anniversary of the birth of Ukrainian poet Taras Shevchenko and patriotic rallies were planned in Kiev and - in a show of defiance towards Russia - in Crimea's key cities of Simferopol and Sevastopol.

The rallies are also significant ahead of controversial plans by Crimea's regional pro-Moscow government to hold a referendum on joining Russia on March 16, which could lead to a formal annexation of the territory.

In the eastern city of Donetsk, a former stronghold of Ukraine's ousted president Viktor Yanukovych and the site of regular protests in recent days, there were fears of clashes as pro-Kiev demonstrators planned to celebrate Shevchenko, while pro-Russian protesters were due to hold a counter-rally.

The 19th-century poet has been credited with creating the modern Ukrainian language and the idea of Ukraine as a nation, giving the new Wester-backed government in Kiev a powerful opportunity on Sunday to revive some nationalist fervour in the face of Russia's incursion.

Late Saturday, Mr Obama made a fresh round of phone calls to key European allies in the latest diplomatic attempts to resolve the Ukrainian crisis, which began with Mr Yanukovych's ouster last month after three months of protests that left some 100 dead.

In talks with his French counterpart Francois Hollande, British Prime Minister David Cameron and Italian leader Matteo Renzi, "the leaders reiterated grave concern over Russia's clear violation of international law and reaffirmed their support for Ukraine's sovereignty and territorial integrity," the White House said.

Mr Hollande's office said the two leaders discussed "new measures" against Russia if it failed to act to defuse the crisis.

Mr Obama also spoke to the leaders of former Soviet states Lithuania, Latvia and Estonia.

Russia's expansive moves, its first military campaign against a neighbour since a brief 2008 conflict with Georgia, have left the Baltic states jittery.

Moscow's promises to "defend" ethnic Russians in Crimea have especially set off alarm bells in Latvia and Estonia, where Russian-speakers make up around a quarter of population.

One 72-year-old man in the Lithuanian capital Vilnius, Gucas, told AFP that Russian President Vladimir Putin was a "madman" who "won't stop until he's stopped by force."

As the crisis has escalated, Washington has imposed visa bans on targeted Russians and Ukrainians and warned of wider sanctions against Russia, while the EU has halted visa and other talks with Moscow.

US Secretary of State John Kerry warned Russia's Foreign Minister Sergei Lavrov again Saturday that "continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia would close any available space for diplomacy."

Mr Lavrov has meanwhile told the West not to take "hasty" action or impose sanctions over Ukraine, warning this would "inevitably have a boomerang effect." A Russian defence official on Saturday said Moscow may halt foreign inspections of its vast nuclear arsenal in response to "threats" from the United States and the North Atlantic Treaty Organisation (NATO).

Adding to the tensions, state-run energy giant Gazprom has warned debt-stricken Ukraine it may cut off gas supplies over an unpaid $1.89-billion bill.

Moscow continues to insist it has sent no troops to Crimea, despite parliament giving Putin the greenlight to do so, and is only deploying units from its Black Sea Fleet stationed there.

Ukrainian Border Guards General Mykola Kovil said there were now 30,000 Russian soldiers in Crimea - 5,000 more than the contingent allowed under a deal with Kiev on the Russian fleet.

On Saturday, OSCE monitors failed for the third time in three days to enter disputed Crimea, as shots were fired in the air as their convoy approached a checkpoint manned by armed men.

The 54 OSCE from 29 countries are a key part of a three-pronged diplomatic push by Obama that includes a call for Russia to pull its Crimean troops back to their barracks and Ukraine to hold early presidential polls in May.

Concern has meanwhile mounted over the safety of journalists in Crimea after reports that Ukrainian reporters were beaten by pro-Russian militants and an international television crew had its equipment seized.