UN Syria mediator Staffan de Mistura said he had put the Geneva peace talks on hold until February 25, pointing to the level of violence happening on the ground.

Before the talks were halted, government forces dealt a major blow to rebels earlier Wednesday, cutting off vital supply lines to and from Aleppo, the last major urban centre in rebel hands.

De Mistura said military action had created an obstacle for the negotiations, which formally opened two days ago.

The indirect talks between the Syrian regime and opposition were often delayed by the parties' bickering and no observable progress had been made.

Syrians needed to see an improvement in humanitarian conditions in tandem with political talks, de Mistura said.

"Since I didn't see that ... I have to be honest with myself and say it's time for a pause, only a pause," he said.

In Syria, state news agency SANA said troops and allied militiamen "completely cut the supply routes of the terrorist organizations" north of Aleppo and lifted the siege on two Shiite villages that had been surrounded by rebels for over three years.

The Syrian Observatory for Human Rights, an opposition monitoring group, confirmed that the route between Aleppo and the Turkish border had been blocked after advances by government forces backed by intense Russian airstrikes.

The development is a significant victory for government forces, who have been trying for over two years to encircle Aleppo. Other rebel-held routes to the northern city remain open.

Opposition delegates to the Geneva talks have appealed for countries backing the rebels to exert pressure on Russia to stop its airstrikes.

But Russian Foreign Minister Sergei Lavrov made it clear earlier that Moscow had no immediate intention of pulling back on its military support for the regime of President Bashar al-Assad.

Speaking to reporters in Muscat, Oman, Lavrov singled out access to the Turkish border as a key issue.

"The key point for the ceasefire to work is a task of blocking illegal trafficking across the Turkish-Syrian border, which supports the militants," he said in comments reported by Russian state news agency TASS.

Lavrov said the aim of Russian operations was "to defeat the Islamic State and Jabhat al-Nusra terrorist groups."

Al-Nusra, the Syrian branch of al-Qaeda, is a key component of the main rebel alliances on the ground in northern Syria. Days ago it sent a large column of fighters to reinforce rebels in Aleppo.

The Geneva talks, officially backed by Russia, the US and other countries variously allied with al-Assad's government and the rebels, are aimed at finding a political solution to the five-year conflict that has devastated Syria and driven half its population from their homes.

The opposition insists on humanitarian measures and an end to Russian and Syrian airstrikes as a first step toward a resolution to the conflict, while the government says the focus must be on combatting terrorism.

In New York, a UN spokesman confirmed that three aid workers of a non-governmental organization have been killed in an airstrike in rural Aleppo in northern Syria.

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