Mr Kerry's comments follow reports that Russia is expanding its military presence in Syria through the development of two additional bases.

Russia's bolstering of its military aid to Syria has concerned US officials.

But Mr Kerry said on Tuesday the US was prepared to work with Russia to end Syria's bitter four-year war.

He urged Russian president Vladimir Putin to play a constructive part in finding a diplomatic solution to the conflict, which has killed more than 200,000 people and displaced millions.

Mr Kerry has been critical of Mr Putin's support for Syrian President Bashar al-Assad, which he said on Tuesday was a factor in motivating foreign fighters to travel to Syria to oppose Mr Assad.

The Wall Street Journal journal reported on Tuesday that new satellite images appeared to reveal Russia expanding its military presence in Syria, near the Mediterranean coast.

The appearance of new Russian fighter jets has raised concerns that Russia intends to play a more aggressive role in the conflict.

"For the moment, it is the judgement of our military and experts that the level and type represents basically force protection," Mr Kerry told reporters.

But he said that Russia's longer-term intentions remained unclear and called on Mr Putin to aid efforts to end the crisis.

"If [Russia is] there to shore up Assad and to certainly provide Assad with the continued sense he doesn't have to negotiate, then I think it's a problem for Syria, and it's a problem for everybody who wants to bring an end to this conflict, which has gone on for too long," Mr Kerry said.

Russia's support for Mr Assad is a headache for Mr Kerry as the US attempts to find a diplomatic solution to the civil war under which the Syrian leader steps down.

UN-led peace talks on Syria have failed to bring the opposing sides together to agree on a transitional government body that would take over from Mr Assad.

But the increased presence of the Islamic State militant group, Russia's military build-up and an exodus of Syrian refugees into Europe have added urgency to finding a political settlement.

Mr Kerry will meet counterparts from Europe and the Middle East on the sidelines of the UN General Assembly next week to discuss solutions to the conflict.

(BBC)