A defiant Russia on Thursday said it was ready to increase military support to Syria's Bashar al-Assad if needed, dismissing suggestions it could hamper Western efforts to fight Islamic State militants.
Washington has accused Moscow of solidifying its foothold in Syria, raising concerns it could throw a wrench in plans by the US-led coalition to step up operations against jihadists in the war-torn country.
The fresh tension erupted just weeks before the UN General Assembly in New York at which Russian President Vladimir Putin is scheduled to speak.
Speaking to reporters, Foreign Minister Sergei Lavrov said Moscow wanted to avoid a repeat of the "Libyan scenario" in Syria and would therefore provide greater military assistance to the Syrian president if requested.
"We helped, are continuing to help and will help the Syrian government when it comes to supplying the Syrian army with everything it needs," he said.
But he denied that Moscow was ramping up its military presence in Syria, saying Russia was merely supplying its ally with humanitarian aid and military equipment in accordance with existing contracts.
"Russian is sending planes to Syria with both military equipment in accordance with current contracts and humanitarian aid," he said.
"We have never made our military presence (in Syria) a secret," he said.
"Russia is not taking any additional steps."
US officials said this week that Russia had been sending ships, armoured personnel carriers and naval infantry to Syria in an apparent effort to prop up the besieged regime.
And he rubbished suggestions any greater Russian involvement in Syria could complicate Western plans to fight Islamic State, which has taken control over swathes of Syrian territory.
"This logic is uncomprehensible to me," Russia's top diplomat said. "The Syrian army is the most effective force that can stand up to the terror threat on the ground."
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On Wednesday, US officials said two tank-landing ships had arrived at Russia's Soviet-era naval facility at the Syrian port of Tartus along with naval infantry.
The White House warned this week that a Russian military buildup in Syria could spark a "confrontation" with US-led forces carrying out an air campaign against the Islamic State group.
-'Upsidedown logic '-
Lavrov said he had discussed Washington's concerns with US Secretary of State John Kerry who had phoned him twice over the past few days.
He said Kerry was concerned that Russia's support for Assad would ultimately strengthen the Islamic State group because its sponsors would be forced to ramp up their military and financial support.
"Well, that logic has been turned upside down," Lavrov said.
"Once again, this is an attempt to appease those who are using terrorists in their fight against unwanted regimes."
"I believe this is a colossal mistake."
Putin has called for the establishment of a wider coalition to combat IS in Syria and his spokesman said he would raise the issue in his UN address.
Russia "sees as its goal giving support to the Syrian authorities in fighting the dangerous phenomenon that is the so-called Islamic State," spokesman Dmitry Peskov said.
Some analysts have suggested Russia is keen to deflect attention from the Ukraine crisis and may use its greater role in Syria as a bargaining chip to ease Western sanctions.
Syria was a close ally of Moscow's during the Soviet era, receiving military backing at a time when Russia supported the Arab world in its confrontation with Israel.