A Syrian government minister has warned that any foreign intervention in the country would be an act of aggression unless it is approved by Damascus, after the United States said it was prepared to strike against Islamic State fighters in the country.

Syria has repeatedly warned that any action on its soil needs its approval and has said it is willing to work with any country to tackle IS fighters who have captured large areas of Syria and neighbouring Iraq.

"Any action of any type without the approval of Syrian government is an aggression against Syria," Ali Haidar, minister of national reconciliation affairs, told reporters in Damascus on Thursday.

"There must be cooperation with Syria and coordination with Syria and there must be a Syrian approval of any action whether it is military or not."

Foreign countries could use the Islamic State threat simply as a pretext for attacking Syria, Haidar told reporters ahead of a meeting with new international peace mediator Staffan de Mistura.

US President Barack Obama authorised late on Wednesday air strikes against Islamic State group targets inside Syria for the first time, pledging to destroy its fighters "wherever they exist".

In an address to the nation, Obama also announced an expansion of strikes in Iraq, saying he would be dispatching nearly 500 more US troops to the country to assist its besieged security forces.

 Obama outlined the strategy against IS on Wednesday

Obama called on Congress to authorise a programme to train and arm rebels in Syria who are fighting both the Islamic State group and Syrian President Bashar al-Assad.

Syria's main Western-backed opposition group welcomed Obama's decision, but while repeating its demand that Assad must go.

"The Syrian Coalition ... stands ready and willing to partner with the international community not only to defeat ISIS [Islamic State] but also rid the Syrian people of the tyranny of the Assad regime," said Hadi al-Bahra, head of the coalition.

Russia said unilateral US strikes in Syria would be a crude violation of international law.

"The US president has directly announced the possibility of strikes by American armed forces against positions of the Islamic State in Syria without the consent of its legal government," said Alexander Lukashevich, a spokesman for the Russian foreign ministry.

"In the absence of an appropriate decision of the UN Security Council, such a step would become an act of aggression, a crude violation of the norms of international law," he said.

Also on Thursday, the foreign minister of Germany said his country would not be taking part in air strikes in Syria, while David Cameron's spokesman said the British prime minister was not ruling out military action against IS.

"In terms of air power, the prime minister has not ruled anything out and that is the position," He said.

Source: Agencies