The United States chided Russia on December 28 for killing a top Syrian rebel leader in an air strike last week, saying the move will make it more difficult to reach a peace deal in Syria.
Zahran Alloush, the leader of Jaysh Al-Islam who commanded thousands of fighters in the Damascus suburbs, was killed December 25.
Jaysh Al-Islam was a leading signatory to an agreement forged in Riyadh among Syrian opposition groups willing to start talks with the Syrian government next year aimed at ending the civil war.
The group warned afterwards that Alloush's death might derail the peace talks, and many analysts agreed that the loss of a leader with his large following and status as a relatively moderate Islamist will haunt the peace negotiations.
"It doesn't send the most constructive message to carry out a strike like that," said U.S. State Department spokesman Mark Toner.
"The strike on Alloush and others in Jaysh Al-Islam and other opposition groups do in fact complicate efforts to bring about meaningful political negotiations and a nationwide cease-fire."