Bulgarian officials said they had been instructed by Moscow to consider using Russian forces to take part in the war in Syria and the annexation of Crimea in Ukraine, as the United States and the European Union seek to prevent Russia from further destabilising the region.
The warning from the government came after Bulgaria announced a series of measures to tighten its military posture amid Russia’s actions in Ukraine.
Bulgaria’s prime minister said that it was ready to defend its borders against any attack by the “armed forces of the USA” but also urged the US to “stop interfering in the affairs of other countries”.
The Bulgarian government has warned it could use force if Washington refuses to back down.
The US, Russia and Britain have all agreed to provide “lethal support” to the Syrian opposition.
US Secretary of State John Kerry said that Washington will continue to support the Syrian government against the “criminal” forces that have turned the country into a failed state.
Mr Kerry said the US and its allies are prepared to provide support “wherever it is necessary” to counter the terrorist threat from “armed opposition groups”.
However, he said that the support was not aimed at the Syrian people but was focused on helping them “protect their own security and the lives of their citizens”.
Mr Kerry was speaking at the G20 Summit in Brisbane, Australia, where the United Nations Security Council has met since September to discuss the crisis in Syria.
Mr Putin told a gathering of the Organisation of Islamic Cooperation in Egypt that the Russian military had the right to respond in any way it saw fit to protect the Russian-speaking population.
Russian President Vladimir Putin said in Cairo that the “security and stability of the whole region depends on the complete support of all the countries of the region”.
“The situation in Syria is totally unacceptable.
We cannot continue to stand by while it is being brought about,” he said.
Mr Obama said Russia had violated the sovereignty of other states in the Middle East and was committing a crime against humanity.
“The United States stands firmly behind the Syrian regime and will not hesitate to defend ourselves and our allies against any aggression,” he added.
The president also said the international community must act now to prevent the return of violence to Syria.
“There is a chance that if we have the right circumstances we can stop the spiral of violence in Syria,” he told a news conference in New York.
“We need to get the violence out of Syria.
We can do that through the use of all of the tools we have, but also through the international institutions that have been established,” he warned.
He said Russia was not in the position to defend itself alone, and that the US had a duty to act to support it.
He also told the gathering of world leaders that the United Kingdom had to act in a way that supported its own security.
“This is not the way we do things,” he claimed.
Mr Lavrov said he would like to see the US, the United Arab Emirates, Saudi Arabia, Egypt and Jordan all work together to create a new Syrian government.
“I hope we will find a solution to this problem.
We need to put all of these states in a situation where they can work together and do everything possible to find a new solution,” he insisted.
Mr Trump on Thursday also announced a $300m arms sale to the Saudis and another $200m to Jordan, as well as a new $1bn deal to buy Russian SU-34 warplanes.
But Russia, which has denied any role in the Syria conflict, rejected the Russian offer, calling it “absurd”.
Russian Foreign Minister Sergei Lavrov said on Thursday that the new US arms deal was “absundant”.
Russian Defence Minister Sergei Shoigu told reporters that the deal should be looked at in its entirety, but added that the Russians had not received any US arms in the past five years.
He denied that the Syrian conflict was being used as a pretext for military aid.
Russia and the US have agreed to cooperate on the fight against Islamic State in Syria but the Kremlin has been less forthcoming on its plans for a broader alliance against the group.