U.S. President Barack Obama arrived in Sweden Wednesday for a two-day visit likely to revolve around Syria, despite the host nation's effort to draw attention to a broader agenda.

Obama stepped off Air Force One at Stockholm's Arlanda Airport fresh from efforts in Washington to secure bipartisan support for military strikes against Syria to punish the regime for what the U.S. says was the use of sarin gas on a Damascus suburb.

U.S. President Barack Obama won strong backing from key Republican leaders Tuesday for military strikes against Syria, as Washington dramatically closed ranks to send a message to President Bashar Assad that chemical weapons must not be used.

Obama told congressional leaders that Assad must be held to account, noting that he was confident he will win "prompt" Senate and House votes on authorizing military action which he wants next week, and pledged to upgrade U.S. help to the Syrian opposition over time.

President Barack Obama and Japanese Prime Minister Shinzo Abe discussed the alleged chemical weapons attack in Syria, agreeing that use of the banned arms cannot be tolerated, the White House said Monday.

"The two leaders agreed that the use of chemical weapons is a serious violation of international norms and cannot be tolerated," it said in a statement.

The visiting head of Iranian parliament's foreign policy committee discussed on Tuesday with top Lebanese officials and Hizbullah leader Sayyed Hassan Nasrallah the latest developments in the region.

Alaeddin Boroujerdi, who is the chairman of the committee for foreign policy and national security of the Iranian parliament, arrived in Beirut on Monday and went into separate talks with Speaker Nabih Berri and Premier-designate Tammam Salam.

Pro-Syrian regime hackers posted messages on a U.S. Marine Corps recruiting website on Monday, urging troops to defy orders from President Barack Obama.

The hackers showed photos of people in American uniforms holding hand-written signs saying they would not fight for al-Qaida in Syria.

The NATO commander in Afghanistan has warned that the current casualty rates suffered by the Afghan army and police force are "unsustainable", in an interview published in Tuesday's Guardian.

U.S. General Joseph Dunford told the paper that Afghan security forces may need five more years of western support before being able to take over full responsibility.

Brazil and Mexico summoned U.S. ambassadors on Monday to demand explanations over allegations that the National Security Agency spied on their presidents' communications.

Brazil's Foreign Minister Luis Figueiredo said the interception of Internet data from President Dilma Rousseff reported by U.S. journalist Glenn Greenwald, if proven, "represents an unacceptable and unallowable violation of Brazilian sovereignty."

Senator John McCain said Monday that Congress' failure to authorize military action in Syria would be "catastrophic" because it would undermine U.S. credibility.

"It would undermine the credibility of the United States of America and the president of the United States. None of us want that," he said after meeting with President Barack Obama.

Former Soviet leader Mikhail Gorbachev said Monday U.S. President Barack Obama was right to seek backing from Congress for military action in Syria, but warned that intervening raised dire risks.

Obama has faced criticism for alleged indecisiveness and for passing the ball to U.S. lawmakers after previously taking a hawkish tone over an alleged chemical attack in Syria.

U.S. Secretary of State John Kerry will join the 28 EU foreign ministers meeting Saturday in Vilnius to discuss a series of issues, notably the Syrian conflict, officials said.

EU foreign affairs head Catherine Ashton said in an invitation letter to ministers that Kerry's attendance "will be a valuable opportunity to exchange views with him on key issues."