BEIRUT, Lebanon — Russian and Syrian officials on Friday strongly condemned an American airstrike on pro-Syrian government forces in southern Syria a day earlier, calling it an act of aggression and rejecting the United States’ justification for the attack.

“It is illegitimate, it is unlawful and the latest gross violation of the sovereignty of the Syrian Arab Republic,” Russia’s foreign minister, Sergey V. Lavrov, said in Cyprus, according to the state-run Russian news agency Tass.

After the airstrike, which American officials said was defensive in nature, Pentagon officials said on Friday that they were working with Russia to prevent similar ground incidents in the future, much as the two countries have done for months to avoid accidents in the skies over Syria.

“We had a proposal that we’re working on with the Russians right now,” Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, told reporters on Friday, declining to give details. “My sense is that the Russians are as enthusiastic as we are to deconflict operations and ensure that we can continue to take the campaign to ISIS and ensure the safety of our personnel.”