Russia warned the U.S. on Monday against stepping up support for Syria's opposition, saying it would only bolster a caliphate declared by the Islamic State in Iraq and the Levant, or ISIL, in part of Iraq and Syria.
Baghdad called for international help as it is struggling to contain an offensive by the ISIL, an al-Qaeda splinter group, which overran Iraq's Mosul earlier in June. It also captured territory in the north and east of Syria.
"The jihadists have challenged not only their competing extremist groups, hiding under the slogans of Islam, but also the countries of the Islamic world," the Russian Foreign Ministry said in a statement.
Five Russian Sukhoi Su-25 fighter jets arrived in Baghdad late on Saturday and Iraqi state television said they would be used to fight the ISIL. Washington also dispatched military advisers to help Iraq stamp out a renewed militant threat.
But in neighboring Syria, Moscow and Washington have thrown their weight behind rival forces in an uprising that started early in 2011 with Russia siding with Syrian President Bashar Assad and the U.S. with those seeking to oust him.
The Russian ministry criticized as "more than strange" a call to the U.S. Congress by U.S. President Barack Obama for $500 million to train and equip "those whom the White House calls 'moderate' units of the armed Syrian opposition".
"Given the real situation on the ground, these considerable funds, if allocated, would in fact serve entirely to strengthen the terrorist 'caliphate' proclaimed by the ISIL," it said.
"Under the circumstances in which terrorist and extremist groups have secured their dominant role among anti-governmental forces in Syria, the best arms and best prepared fighters would end up with them."
Defending Obama's move, U.S. Secretary of State John Kerry on Friday said the moderate Syrian opposition had a key role in repelling the ISIL.
Separately, Russian Foreign Ministry spokesman Alexander Lukashevich, when asked on Monday about the Su-25 deliveries to Iraq, said Russian arms supplies to Baghdad were now completed but gave no more detail.