U.S. President Barack Obama on Thursday spoke with German Chancellor Angela Merkel about the situation in Syria, the White House said, as the West weighs military action against Damascus.

"The president today conducted a phone call with German Chancellor Angela Merkel. This is part of the series of communications that the president has initiated around the situation in Syria," spokesman Josh Earnest told reporters.

Western plans for retaliatory action against Syria for an alleged chemical weapons attack are "difficult to develop", the French government spokeswoman said Thursday.

"The international community must find a riposte that is adapted to the situation," Najat Vallaud-Belkacem said on France 2 television.

President Barack Obama said Wednesday he had not yet signed off on a plan to strike Syria, but action appeared likely after Washington abandoned the hunt for a last-minute U.N. mandate.

Political uproar in London, meanwhile, cast doubt on whether Britain will join American military action to punish President Bashar Assad's regime for a chemical weapons attack, should the response take place before next week.

Ten years after George W. Bush brushed aside the U.N. Security Council to invade Iraq, U.S. President Barack Obama is poised to strike Syria without U.N. approval -- while insisting this time it's different.

With zero chance of the U.N. Security Council backing a British resolution calling for "measures" against Syria's President Bashar Assad, there is talk of a "coalition of the willing", reminiscent of the group gathered by Bush to oust Iraqi dictator Saddam Hussein.

President Barack Obama will lead his nation in homage Wednesday to Martin Luther King, at the spot where the civil rights icon voiced a soaring dream of equality 50 years ago.

In a moment of high symbolism, America's first black president will reflect on King's legacy and the long march, which still beckons to fulfill the hopes of the "I have a dream" speech, delivered from the steps of the Lincoln Memorial in Washington.

Any military intervention by the United States against Tehran's ally Syria will spell "disaster" for the region, Iranian supreme leader Ayatollah Ali Khamenei warned on Wednesday.

"The U.S. intervention will be a disaster for the region," Khamenei, the Islamic republic's most powerful authority, told a meeting with the cabinet of President Hassan Rowhani, state television reported.

President Barack Obama teared up watching "Lee Daniels' The Butler," a celluloid depiction of the U.S. civil rights battle as seen by an African-American who served in the White House.

The movie, starring Forest Whitaker and Oprah Winfrey, topped the North American box office for a second straight weekend and is already being spoke of as a multiple Academy Award contender.

British Prime Minister David Cameron's office said Wednesday that he and U.S. President Barack Obama had no doubt Syrian President Bashar Assad's regime had used chemical weapons against its own people.

"Both leaders agreed that all the information available confirmed a chemical weapons attack had taken place, noting that even the Iranian President and Syrian regime had conceded this," Downing Street said after the two leaders spoke on the phone Tuesday night.

Dozens of U.S. lawmakers are calling on President Barack Obama to consult with Congress and gain its approval before intervening militarily in Syria.

The United States is poised to launch a military strike on Syria in coming days after determining that President Bashar Assad's regime used chemical weapons during an attack outside Damascus last week which killed hundreds of civilians.

U.S. President Barack Obama is weighing an attack against Syria that would be of limited scope and duration, newspapers reported Tuesday.

The strike in retaliation for what the United States says is Syria's undeniable use of chemical weapons against civilians would probably last no more than two days and seek to keep America out of wider involvement in the country's 29-month civil war, the Washington Post reported, quoting unnamed senior administration officials.