DAMASCUS (Reuters) - Any foreign intervention in Syria would be an act of aggression unless it is approved by Damascus, a Syrian government minister said on Thursday, after the United States said it was prepared to strike against Islamic State militants 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 Islamic State fighters who have captured large areas of Syria and neighboring 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.

"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 Islamic State simply as a pretext for attacking Syria, Haidar told reporters ahead of a meeting with new international peace mediator Staffan de Mistura.

Washington and its Western allies have supported the insurgency against Syrian President Bashar al-Assad and have ruled out cooperating with him to fight Islamic State, which seeks to create a cross-border caliphate.

Instead the United States wants to bolster moderate Syrian rebels fighting Assad and has asked Congress to authorize $500 million to train and arm them, and has said it is willing to carrying out air strikes against Islamic State targets in the country.

In Washington on Thursday State Department spokeswoman Marie Harf told reporters that any military action Washington was considering inside Syria would be aimed at the Islamic State militants and not at the Assad government.

President Barack Obama "has emphasized repeatedly that Assad had lost all legitimacy and must go, but the President has also been clear and his first priority is the safety of the American people. ISIL poses a threat to us and that is what we’re focused on when it comes to any potential action in Syria," she said, using an alternative acronym for the Islamic State.

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She said there needed to be a political transition in Damascus: "There cannot be a military solution for the Assad regime leaving power."

Syria's Western-backed National Coalition opposition has said it is ready to work with the United States against Islamic State, saying it had long called action and had repeatedly warned about the threat of radical Islamists.

Speaking in Damascus after meeting with Assad and other senior Syrian officials, U.N.-appointed mediator Mistura said it was important for the international community to tackle radical militant groups in Syria.

"The terrorist threat has become an international concern shared by everyone," he told a news conference, adding that the international community was getting closer to taking action.

He said he had a "very long and useful meeting" with Assad, but declined to give details, saying only that the United Nations would work with Syria to help tackle the violence, provide humanitarian aid and work on a solution to the crisis.

"Syrians, wherever they are, and the government should be helped by the United Nations and the international community to find a Syrian-owned all-inclusive, positive, political process," he said.

Syria has said it is willing to work with Mistura and has called on him to be fair and respect the wishes of the Syrian people.

(riting by Sylvia Westall and David Storey; Editing by Alison Williams and James Dalgleish)