VIENNA — Secretary of State John Kerry and his Russian counterpart, Sergey V. Lavrov, said on Tuesday that if President Bashar al-Assad of Syria continues to block access of humanitarian aid to besieged cities and towns, they were prepared to help the World Food Program airdrop food and emergency supplies.

The very fact that they had to threaten the airdrops — which are expensive and often inaccurate — amounted to an admission of how little progress has been made in achieving either the lasting cease-fire or the regular humanitarian relief that European and Arab nations, along with Iran, laid out as the first steps toward a broader peace agreement.

The threat to conduct airdrops came after a meeting in Vienna of the International Syria Support Group, made up of the nations that drafted a largely unimplemented plan to end the country’s civil war. They gathered at a low point: A once-promising “cessation of hostilities” has largely collapsed, an effort to start negotiations between the opposition and the government broke down, and there has been no progress toward negotiating a “political transition” that was supposed to begin on Aug. 1.

Bolstered by Russia’s intervention to help prop him up, Mr. Assad is in a stronger position than he has been in years, many experts say, and has rejected the idea that any new government would have to exclude him. He has the strong support of Iran, his longtime provider of security, though Russian officials seem less concerned about whether Mr. Assad himself remains in power or is replaced by another leader from his Alawite Shiite sect.