WASHINGTON - President Barack Obama, senior cabinet officials and his defense and intelligence chiefs will meet with US lawmakers this week and next as the administration seeks support for an offensive against Islamic State militants.

Obama will deliver a speech to Americans on Wednesday to lay out his "game plan" to halt the Islamist group that has seized swathes of Syria and Iraq, as he tries to head off public concerns that the country could be moving toward another full-scale war.

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Ahead of the speech, Obama will meet on Tuesday with the four leaders of Congress - Senate Majority Leader Harry Reid, Senate Minority Leader Mitch McConnell, Speaker of the House John Boehner and House Minority Leader Nancy Pelosi.

Also on Tuesday, his CIA Director John Brennan, and James Clapper, the US Director of National Intelligence, will brief the Senate Intelligence Committee, the panel's chairwoman, Senator Dianne Feinstein, told reporters.

Administration officials also will hold a briefing for the 100 members of the Senate on Wednesday and for all 435 members of the US House of Representatives on Thursday.

Secretary of Defense Chuck Hagel and General Martin Dempsey, chairman of the Joint Chiefs of Staff, will testify on Sept 16 at a hearing on US policy toward Iraq and Syria and the threat posed by the Islamic State.

And John Kerry, the US Secretary of State, is due to testify the same day before the House Foreign Affairs Committee.