KIEV (AFP) - The leaders of Ukraine, Germany, France and Russia on Thursday pledged renewed support for a tattered ceasefire in eastern Ukraine despite violations - including the storming of a key town by pro-Russian rebels.

As the leaders condemned the fighting and urged both sides to observe the truce, there was strident opposition from the separatists and Moscow to a plea from Ukraine for international peacekeepers to enforce the ceasefire.

Washington said pro-Russian rebels had broken the ceasefire more than 250 times since it came into force on Sunday.

The Ukrainian army, meanwhile, said 90 troops had been captured and 82 were still missing after the rebels seized the strategic town of Debaltseve.

The seizure of the town, a transport hub sandwiched between the rebel strongholds of Donetsk and Lugansk, sent government troops into retreat.

The insurgents claimed to have captured as many as 300 government soldiers.

One of the rebel leaders, Alexander Zakharchenko, said that 3,000 to 3,500 troops died in the assault, although such casualty counts on both sides are often greatly exaggerated for propaganda effect.

"Let Kiev take their dead," he said.

- Peacekeeper call -

Kiev's defeat in Debaltseve, which has had many Ukrainians questioning the competence of their military leaders, prompted Ukrainian President Petro Poroshenko to appeal for international peacekeepers to be deployed in the east.

Poroshenko again raised the proposal in a four-way telephone conversation on Thursday with German Chancellor Angela Merkel, French President Francois Hollande and Russian President Vladimir Putin, his office said.

He found little apparent support, however, with none of the others mentioning it in statements from their offices, and Moscow denying it was mentioned at all.

Instead, the four called for the implementation of the full package of measures agreed in Minsk, including a full ceasefire, withdrawal of heavy weapons and the release of prisoners, according to the French presidency.

They also called for observers from the Organization for Security and Cooperation in Europe (OSCE) to be able to carry out their task of monitoring the ceasefire.

The OSCE, which rebels have blocked from entering Debaltseve, said there were "no exceptions" to the ceasefire's application in the conflict zone - explicitly rejecting the separatists' argument that Debaltseve should be excluded.

Journalists have also been prevented from getting into the war-ravaged town.

An AFP reporter about four kilometres from Debaltseve on Thursday saw rebels firing Grad rockets for 15 minutes, making the earth tremble.

"There's still fighting, there has been shelling, it's risky," a rebel commander appointed as the new mayor of the town, Alexander Apendikov, said as his convoy stopped on the road.

The commander, nicknamed "The Greek" for his ethnic origins, said the insurgents were going building to building in Debaltseve looking for remaining Ukrainian soldiers refusing to surrender.

"They are hiding, some of them in the woods, we don't know what they're going to do," he said.

- Russia accused -

The EU and US have called the rebel assault of Debaltseve a "clear violation" of the truce aimed at quelling Ukraine's 10-month-old conflict, which has killed more than 5,600 people.

"This violence has continued despite Russian and separatist commitments to a ceasefire agreement," White House deputy press secretary Eric Schultz said Thursday. "We call on all parties, including Russia, to abide by that plan."

As Western powers pledged support for the ceasefire agreement, a scathing report from a British parliamentary committee said Britain and the EU were "guilty of sleepwalking into this crisis".

The EU Committee of the House of Lords said the EU's relationship with Moscow had long assumed an "optimistic premise" that Russia was on the path to becoming more democratic.

"The lack of robust analytical capacity, in both the UK and the EU, effectively led to a catastrophic misreading of the mood in the run-up to the crisis," said committee chairman Christopher Tugendhat.

In Donetsk, intense artillery fire continued throughout the day, according to a journalist on the ground.

A UN convoy carrying humanitarian aid arrived in the town Thursday, the first to arrive in the rebel stronghold.

The West has accused Russia of sending troops, tanks and weapons to support the separatists with the aim of keeping Ukraine, an ex-Soviet republic, too destabilised to move closer towards the EU and Nato. Moscow denies the allegation.

US Secretary of State John Kerry told his Russian counterpart Sergei Lavrov on Wednesday "to stop Russian and separatist attacks on Ukrainian positions in Debaltseve and other violations of the ceasefire".

President Barack Obama has also been mulling sending arms to Ukraine if the situation worsens.

But Russia has a mighty military that could trump any armed escalation in Ukraine, and is an important gas supplier to several EU countries. It has already retaliated against sanctions by banning European food imports.

Britain's Foreign Secretary Philip Hammond said on Thursday that his country remained opposed to arming Ukraine, but noted the question was "one for individual national governments".