Almost 5,000 people were killed in Syria in January alone, according to new figures reported on Sunday which underscored the escalation of hostilities in a country thousands are fleeing every day.

The latest death toll – the second-worst month in the two-year conflict – was reported by the UK-based Syrian Observatory for Human Rights, a dissident group whose casualty estimates have been consistently confirmed by the UN. Its director, Rami Abdulrahman, said his researchers had recorded the deaths of 4,851 people in January, of whom 1,030 were members of the Syrian regular security forces while 3,305 were civilians or rebel irregulars.

The dismal figures – which come amid a growing refugee crisis in the border camps which are overrun with people fleeing the conflict – underlined the urgent need to find some form of diplomatic breakthrough. At the weekend, for the first time, the foreign ministers from Russia and Iran, the Assad regime's closest international supporters, met the Syrian opposition leader, Moaz al-Khatib, in a rare sign of diplomatic progress.

Following Khatib's offer to hold preliminary talks with the regime, conditional upon the release of political prisoners, the discussions at a global security conference in Munich raised hopes that a way could be found around the deadlock in the UN security council.

After his meeting with the Russian foreign minister, Sergei Lavrov, the Syrian opposition leader said: "Russia has a certain vision but we welcome negotiations to alleviate the crisis and there are lots of details that need to be discussed."

The Iranian foreign minister, Ali Akbar Salehi, said Iran would hold further meetings with Khatib. He called for the formation of a joint transitional government to include members of the regime and the opposition, under UN supervision, leading to elections and a new constitution.

However, there was no sign of a breakthrough over the sticking point that has divided the security council and prevented Syrian peace talks: the fate of the president, Bashar al-Assad.

Lavrov told the Munich conference: "The persistence of those who say that priority number one is the removal of Assad is the single biggest reason for the continuing tragedy in Syria."

Salehi was less specific. He made no mention of Assad, but he said: "If you ask for the government to stand down before negotiations, who do you negotiate with?"

On Saturday, the US vice president gave his full support to the opposition stance that Assad has so much blood on his hands he could not be part of a transition government. Joe Biden said the White House was "convinced that President Assad, a tyrant hell-bent on clinging to power, is no longer fit to lead Syrian people and he must go".

Moscow has become increasingly isolated in its backing for Assad. Lakhdar Brahimi, the UN envoy, told the security council last week that the implication of an agreement of major powers last year in Geneva was that Assad should have no part in the transition process.

The Turkish foreign minister, Ahmet Davutoglu, ridiculed the idea that the Syrian leader should remain in power to oversee a transition, .

"It's easy to say the opposition should sit down with him now after 60,000 people have been killed," Davutoglu said. "If they held an election in his presence who would guarantee the security of the opposition? There should be an election, but first someone should be [held] responsible for all the killing."

The Qatari prime minister, Sheikh Hamad bin Jaber al-Thani, said repeated attempts to organise talks between Assad and the opposition in the early months of the Syrian uprising had failed because of "the intransigence of the regime".

"I have no doubt Assad will leave, because he cannot stay with so much blood on his hands," he said. He also criticised Israel for its air strikes in Syria last week, which he said would "add fuel to the fire".

In the first direct comment by an Israeli official on Tuesday's air strikes, Ehud Barak, the outgoing defence minister and deputy prime minister, appeared to confirm widespread reports that it was targeted at anti-aircraft missiles bound for Hezbollah in Lebanon.

"What happened in Syria several days ago … that's proof that when we said something we mean it, we say that we don't think it should be allowed to bring advanced weapons systems into Lebanon," Barak told the Munich conference.

Bashar al-Assad said on Sunday that his military was capable of confronting any "aggression" that targeted the country, in his first remarks since the Israeli strike.

The Syrian Observatory's estimate of the total number of dead from almost two years of conflict is 51,167. That is below the UN estimate of 60,000, but the Observatory's methodology is more conservative, requiring confirmation of the names of the dead. Of that total, 3,717 of the war's victims were children and 2,144 were women.