BEIRUT, Lebanon — Syrian peace talks in Geneva were overshadowed on Tuesday by a newly launched government offensive in Aleppo that threatens to cut off a key rebel supply line.

Talks in Switzerland between the political and military opposition to President Bashar al-Assad and the Syrian government officially opened on Monday, but the two sides have yet to negotiate even indirectly, and any chance of moving forward was threatened by the failure of both sides to agree on basic principles.

In the past couple of days, the Syrian government launched a major offensive north of Aleppo, Syria’s largest city and a key rebel stronghold. Backed by Russian airstrikes, the Syrian army reportedly took control of the towns of Hardatnein, Tal Jibbeen, and Deir Zaitoun, as part of a wider push to encircle the city. The offensive aims to cut off a crucial supply line from the Turkey border to rebel-held areas of Aleppo.

For the Syrian opposition and the rebel fighters it represents, the offensive is being viewed as a sign that the government in Damascus is not serious about pursuing peace, but is instead seeking to use the talks as a means to gain an advantage on the battlefield.

Salim al-Muslat, spokesman for the opposition negotiating body, accused Syria and Russia of trying to derail the talks.

"It is clear from the current situation that the regime and its allies — in particular Russia — are determined to reject the UN's efforts to implement international law," he said. "The regime and Russia's actions gravely threaten the political process at this early stage."

One rebel leader reportedly called for opposition representatives to withdraw from the talks. Another said reinforcements were being sent to the war's front lines: "We sent new fighters this morning, we sent heavier equipment there. It seems it will be a decisive battle in the north God willing," Ahmed al Seoud, head of a Free Syrian Army group known as Division 13, told Reuters. "We sent TOW missile platforms. We sent everything there.”

That Aleppo is being threatened by government forces is a sign of how Russian airstrikes have turned the tide for Assad's forces since they began in late September last year. Ostensibly aimed at weakening the Islamic State, Moscow has in fact targeted the moderate opposition in equal measure. Civilians have also borne the brunt, with more than 1,500 thought to have been killed by Russian bombs, according to monitoring groups, even though Russia routinely denies this. It comes at a time when the Syrian government has made advances in other parts of the country — notably in the northwest.

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Developments in Geneva were as slow as advances in Aleppo were fast. The delegation representing Syrian President Assad said it was too early for indirect talks, while the opposition sought assurances on relieving areas besieged by the government and the release of political prisoners.

"The circumstances on the formalities are not yet ready. We are in the preparatory stage before the official launch of indirect negotiations," Syrian government delegation chief Bashar al-Ja'afari told reporters after meeting with the UN’s Syria envoy Staffan de Mistura.

"To prepare the official launch we have to have the presence of the two delegations, but on the other side the delegation has not been finalized."

Neither side had held out much hope that the negotiations could make any substantial progress to end a war that has killed more than 250,000 people and created millions of refugees. But accusations of bad faith by both sides have threatened to stall them before they have even begun.

Mohammed Alloush, a spokesman for the Army of Islam, one of the largest rebel groups in Syria and part of the opposition negotiating team, told reporters that he saw little chance for progress.

“The situation in the ground has not changed and as long as the situation stays like that there is no optimism from our side and no good intention to reach a solution by the regime," he said.