The leaders of Ukraine, Germany, France and Russia on Thursday pledged to try to save 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.

The Ukrainian army, meanwhile, said 90 troops had been captured and 82 were still missing after the rebels seized the strategic town of Debaltseve, in violation of the ceasefire which came into force after midnight Sunday.

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

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

One of the rebel leaders, Alexander Zakharchenko, alleged 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.

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

Russia and the rebels immediately voiced fierce opposition to such a move, saying it would "destroy" the truce agreements struck in the Belarus capital Minsk.

But Poroshenko again raised the idea in a four-way telephone conversation Thursday with German Chancellor Angela Merkel, French President Francois Hollande and Russian President Vladimir Putin.

He found little apparent support, however, with none of the others mentioning it in statements from their offices.

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.

A statement said foreign ministers from the four countries would speak later Thursday to define how the ceasefire was being implemented.

A French diplomatic source also told AFP there "will be a meeting, maybe in Paris, in coming days".

Poroshenko, however, stressed that the pull-back of heavy weapons from the frontline -- which did not begin on Tuesday as agreed -- could only happen once the ceasefire was deemed to be "comprehensive".

The four leaders also call 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 has complained that the rebels have for days been blocking its team from entering Debaltseve.

It said that there were "no exceptions" to the ceasefire's application in the conflict zone -- explicitly rejecting the rebels' argument that Debaltseve should be excluded.

Journalists have also been prevented from getting into the town.

An AFP reporter who was about four kilometers (over two miles) 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 EU and U.S. have called the rebel assault of Debaltseve a "clear violation" of the truce aiming to quell Ukraine's 10-month-old conflict that has killed more than 5,600 people.

The West accuses Russia of sending troops, tanks and weapons to support the separatists with the aim of keeping Ukraine, an ex-Soviet republic, too destabilized to move closer towards the EU and NATO.

The West has been increasing sanctions on Russia over its perceived role in backing the insurgents, and 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".

U.S. 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.

Moscow appears keen to see compliance with the ceasefire compliance and for the heavy weapons pull-back to go ahead now the rebels have taken over Debaltseve.