U.S. Secretary of State John Kerry suggested Tuesday that Russia and Iran may persuade Syrian strongman Bashar Assad to stop indiscriminately dropping barrel bombs on his own people.

Moscow and Washington disagree sharply about Assad's longer term role in efforts to end the Syrian war, but have begun cautiously to discuss efforts to defeat his Islamic State jihadist foes.

Moscow on Tuesday slammed the holding of a U.S.-led counter-terrorism summit at the United Nations, calling it disrespectful.

On Tuesday, President Barack Obama sat down with more than 100 leaders at the U.N. to push ahead with a U.S.-led campaign against the Islamic State group.

Lithuanian President Dalia Grybauskaite on Tuesday slammed her Russian counterpart Vladimir Putin for a speech he delivered at the United Nations General Assembly that she said was reminiscent of Soviet-era totalitarianism.

The outspoken leader of the formerly Soviet-ruled, EU state said Putin's criticism of the West suggested he "was still living in the 20th century".

U.S. President Barack Obama said Tuesday that defeating the Islamic State group in Syria will only be possible if Bashar Assad leaves power, a day after a clash with Russia over the Syrian president's fate.

"In Syria (...) defeating ISIL requires, I believe, a new leader," Obama told a counter-terrorism summit of some 100 leaders, held on the sidelines of the U.N. General Assembly.

French Foreign Minister Laurent Fabius accused Russia Tuesday of displaying bravado on the Syria crisis that had yet to be backed up with action against the Islamic State group.

"You have to look at who is doing what. The international community is striking Daesh. France is striking Daesh. The Russians, for the time being, are not at all," Fabius told a news conference in New York, using the Arabic acronym for IS.

Ukrainian pilot Nadiya Savchenko, on trial for the murder of two Russian journalists, said Tuesday she was captured by pro-Kremlin separatists and then smuggled into Russia by armed, masked men.

"I am a prisoner of war and a hostage who has been abducted," Savchenko told the court in a dramatic speech, saying separatist rebels captured her in eastern Ukraine and told her they would "sell" her to Russia.

Moscow said on Tuesday that Russia and the United States intended to build upon a key meeting between Vladimir Putin and Barack Obama and look for ways to fight the Islamic State group.

The Russian president stole the show at the U.N. General Assembly on Monday, calling for a broad U.N.-backed coalition to fight Islamic State jihadists.

Ukraine on Tuesday launched the criminal trial of two captured Russian soldiers it claims will prove Moscow's direct but covert involvement in the conflict in the war-torn former Soviet state.

Kiev's forces seized the two wounded men during a May gun battle that violated a ceasefire the warring sides had signed up to under stronger European pressure three months earlier.

Russian President Vladimir Putin has stolen the spotlight at the United Nations with a swaggering push on the Syria crisis as he tried to shake off Western isolation over Ukraine.

In his first speech to the UN General Assembly in a decade, Putin on Monday called for a broad UN-backed coalition to fight Islamic State (IS) jihadists before sitting down to talks he called "constructive, business-like and surprisingly open" with his U.S. rival Barack Obama.

Russian President Vladimir Putin and his U.S. counterpart Barack Obama met Monday on the crisis in Syria but failed to resolve their dispute over the future role of Bashar Assad.

In dueling speeches before the U.N. General Assembly, Obama branded the Syrian leader a child-killing tyrant while Putin said the world should support Assad against the Islamic State group.