The top U.S. military officer told Congress on Thursday that he believes Russia bombed a humanitarian aid convoy in Syria earlier this week, killing 20 civilians in an "unacceptable atrocity," as the bitter feud between the two nations over the five-year Syrian civil war escalated further.

The statement from U.S. Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, marked the first time a senior U.S. official has publicly pointed the finger at Moscow for the incident. Russia's defense ministry quickly disputed Dunford's comments.

The testimony from Dunford and Defense Secretary Ash Carter, who also appeared before the panel, underscored the Pentagon's mistrust of Moscow and highlighted the hurdles of forging a lasting truce in Syria, where the war has killed nearly 500,000 people and allowed the Islamic State group to develop into a global threat.

Under questioning from the Senate Armed Services Committee, Dunford revealed for the first time that both Russian and Syrian aircraft were in the area at the time of the strike. And when pressed for his own conclusion, Dunford said he believed Russia launched the airstrike.

U.S. officials initially said the aircraft that dropped the bombs were Russian Su-24 fighter jet but that they weren't sure if the aircraft were piloted by Russian or Syrian government troops. The Obama administration has blamed Russia either way, because of Moscow's continued influence over the Syrian government in the war.

When first asked if Russia bombed the aid convoy, Dunford said it wasn't certain which aircraft dropped the bombs. Under further questioning from Sen. Lindsey Graham, R-S.C., on whether it was Russia, Dunford answered, "that hasn't been concluded, but my judgment would be that they did." He was later asked again if it was Russia, and he answered yes.

Maj. Gen. Igor Konashenkov, a spokesman for Russia's defense ministry, called Dunford's testimony his "personal opinion" and contended that the U.S. fears having to acknowledge responsibility for the attack.

The ministry had denied any Russian or Syrian involvement and also suggested a U.S. coalition Predator drone was operating nearby when the convoy attack occurred. The Pentagon said no drone was in the area at the time.

At the White House, spokesman Josh Earnest heaped on further criticism for what it called Moscow's failure to ensure Syria lived up to its commitments under the cease-fire. "That creaking sound you hear is Russia's international credibility taking an additional hit," Earnest said.

Dunford and Carter faced Republicans angry that the Obama administration is not taking more aggressive steps to end the war in Syria. Sen. John McCain, R-Ariz., the committee chairman, accused Secretary of State John Kerry of being "intrepid but delusional" for trying to work with Russia to secure long-term peace in Syria.

A Sept. 9 temporary truce in Syria brokered by the U.S. and Russia has all but collapsed. The cease-fire envisioned a military partnership between the two countries against the Islamic State and al-Qaida if violence was reduced and aid delivered over the course of seven continuous days. The Pentagon, however, voiced reservations about coordinating airstrikes and sharing intelligence with Russia.

Dunford told the committee he doesn't believe that "it would be a good idea to share intelligence with the Russians."

Kerry called for all warplanes to halt flights over aid routes, and at a U.N. Security Council session, he raised "profound doubt" about the willingness of Russia and Syria to abide by the cease-fire.

Carter and Dunford both disagreed with a Kerry proposal to ground all warplanes. They told the committee the U.S. should not discontinue flying aircraft over Syria. Dunford said he sees "no reason to ground our aircraft" and that the U.S.-led coalition needs to maintain pressure on the Islamic State group.

During the wide-ranging hearing, Dunford also said the Obama administration is considering directly arming the Syrian Kurds whom the U.S. has been backing in the fight against the Islamic State.

"They are our most effective partner on the ground," Dunford said of the battle-hardened Kurds, who are senior partners in an opposition group the U.S. calls the Syrian Democratic Forces.

Dunford said it "is very difficult" to manage the balance between supporting the Kurds and dealing with the Turkish government's adamant opposition to that support. Turkey sees the Kurds as a long-term political threat.

Asked whether arming the Syrian Kurds is a viable military option, despite the Turkish government's opposition, Dunford said "reinforcing" the Syrian Democratic Forces' military capabilities "will increase the prospects of our success" in enabling the recapture of Raqqa, the defacto Islamic State capital in Syria.

Dunford also said during an exchange with Graham that Syrian President Bashar Assad "is in a much stronger place than he was a year ago."