MUNICH -- U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov opened talks Thursday to try and reconcile deep differences over a proposed ceasefire for the Syrian civil war. They met as other U.S. and Russian officials traded allegations over bombing of civilian areas in the besieged city of Aleppo.

Russia has proposed a March 1 ceasefire, but Washington believes that will only give Moscow and the Syrian government three weeks to crush moderate rebel groups. The U.S. has countered with a demand for an immediate truce.

As the meeting began at a Munich hotel, Kerry declined to speculate on whether an agreement could be reached.

"We're going to have a serious conversation about all aspects about what's happening in Syria," he said. "Obviously, at some point in time, we want to make progress on the issues of humanitarian access and cease-fire. We will talk about all aspects of the conflict."

Lavrov said that Russia had already submitted a "quite specific" proposal, and "we will wait for the American response before we take it to the ISSG." The ISSG is the International Syria Support Group, a collection of about 20 countries with interests in the conflict, that was due to meet later Thursday in Munich.

The talk of new cease-fire plans comes as the U.S., Russia and more than a dozen other countries meet to try to halt five years of civil war. The conflict has killed more than a quarter-million people, created Europe's biggest refugee crisis since the Second World War and allowed the Islamic State to carve out its own territory across parts of Syria and neighbouring Iraq.

Russia has said it is supporting Syrian President Bashar Assad's government as part of a counterterrorism campaign. But the West claims the majority of Russia's strikes are targeting moderate groups that are opposed to Assad and the Islamic State.

The most recent Russian-backed offensive, near Aleppo, prompted opposition groups to walk out of peace talks last month in Geneva, while forcing tens of thousands of civilians to flee toward the Turkish border.

One U.S. official familiar with the situation said the U.S. can't accept Russia's March 1 cease-fire offer because opposition forces could suffer irreversible losses in northern and southern Syria before the cease-fire ever takes hold.

That official and another said the U.S. counterproposal is simple: A cease-fire that is effective immediately and is accompanied by full humanitarian access to Syria's besieged civilian centres. The officials requested anonymity because they weren't authorized to speak publicly about private diplomatic discussions in the run-up to the Munich conference.

The talks got off to a sour note as Russian and the U.S. officials exchanged accusations over the bombing of hospitals in Aleppo.

The Russian Ministry of Defence rejected a Pentagon claim Wednesday that Russian aircraft hit two hospitals in Aleppo, instead countering that it was U.S. aircraft that operated over the city Wednesday.

Russian Defence Ministry spokesman Maj. Gen. Igor Konashenkov said Thursday that Russian jets hit targets no closer than 20 kilometres away from Aleppo. Konashenkov said that two A-10 ground attack jets of the U.S. Air Force flew in from Turkey Wednesday and attacked Aleppo.

The Baghdad spokesman for the U.S.-led coalition against the Islamic State group refuted the Russian claims and said the incident was an example of the Russian "indiscriminate" use of force.

A senior State Department official also said claims made by Russian officials of U.S. airstrikes in Aleppo were false.

In the latest developments on the ground, Kurdish fighters and their allies captured a military air base in northern Syria under the cover of airstrikes as Russia announced that its air force carried out more than 500 combat missions in Syria over the past week.

Syrian troops launched a wide offensive in northern Syria earlier in February under the cover of Russian airstrikes during which they were able to capture areas from opposition fighters and now are trying to regain rebel-held neighbourhoods in the northern city of Aleppo.

Syria's Kurds have been among the most effective forces battling the Islamic State group, but have remained largely neutral in the conflict between President Bashar Assad and the rebels fighting to overthrow him.

But on Wednesday, the main Kurdish People's Protection Units, or YPG, launched an attack along with some Arab allies against the Mannagh military air base and captured it later in the day, according to a rebel commander and an opposition activist group. The Kurds appeared to be exploiting the chaos to expand their nearby enclave, known as Afrin.

The Britain-based Syrian Observatory for Human Rights said the offensive came as warplanes believed to be Russian carried out 30 airstrikes in the area. It said the YPG and allied forces also captured a nearby village, also called Mannagh.

In Moscow, Konashenkov also said that since Feb. 4, Russian warplanes carried out 510 combat missions and destroyed 888 "terrorist facilities" in several Syrian provinces including Aleppo, Daraa and Latakia where the government offensive is concentrated.

"We have not witnessed such bombardment since the revolution began," said Abu Thabet, referring to the start of the uprising against Assad's government in March 2011. The uprising later turned into a civil war that has killed at least 250,000 people.

Associated Press writer Nataliya Vasilyeva in Moscow, Jamey Keaten in Geneva and Bassem Mroue in Beirut contributed to this report