Secretary of State John Kerry, accompanied by Australian Foreign Minister Bob Carr, gestures as he speaks to reporters following their meeting at the State Department in Washington, Monday, March 18, 2013. (AP Photo/Manuel Balce Ceneta) WASHINGTON (AP) — The Obama administration lent its support Monday to British and French plans to arm Syria’s rebels, saying it wouldn’t stand in the way of any country seeking to rebalance the fight against an Assad regime supported by Russia, Iran and Hezbollah. Secretary of State John Kerry said the longer Syria’s two-year civil war goes on, the greater the danger of its institutions collapsing and extremists getting their hands on the Arab country’s vast chemical weapons arsenal. With some 450,000 Syrians living in neighboring countries as refugees already, he said the conflict is becoming a “global catastrophe.” Kerry said the world needs to change Syrian President Bashar Assad’s calculations. “If he believes he can shoot it out, Syrians and the region have a problem, and the world has a problem,” Kerry told reporters after a meeting with Australian Foreign Minister Bob Carr. Kerry said the U.S. wants to leave the door open for a political solution. But concerning Syria’s rebels, he added, “the United States does not stand in the way of other countries that made a decision to provide arms, whether it’s France or Britain or others.” The comments come after French President Francois Hollande said last week that his country and Britain were pushing the European Union to lift its arms embargo on Syria as soon as possible so that they can send weapons to rebel fighters. The two countries are seeking military help for the rebels by the end of May or earlier if possible. But Germany and other EU nations have been skeptical about sending weapons, pointing to the risk of further escalation in a volatile region. The United States long held the same conviction, with President Barack Obama and other officials saying more weapons in Syria would only make peace harder. As the violence has worsened over the last year, Washington has tempered that message somewhat. It is now promising nonlethal aid to the anti-Assad militias in the form of meals and medical kits, and refusing to rule out further escalation. And support for greater U.S. involvement appears to be growing in Congress. On Monday, the top Democrat on the House Foreign Affairs Committee introduced legislation to train and arm vetted Syrian opposition forces. The groups must be opposed to Assad, willing to establish a peaceful and democratic Syria and committed to securing and safeguarding chemical and biological weapons. No aid could go to a group associated with a foreign terrorist organization, according to the bill by Rep. Eliot Engel of New York. “It is time for us to develop a comprehensive approach to stopping the carnage,” he said in a letter to his colleagues last week. However, the chairman of the Joint Chiefs of Staff, Gen. Martin Dempsey, said Monday he is advising the Obama administration to “proceed cautiously” on Syria, in part because the U.S. is increasingly unclear about the makeup of rebel forces. “About six months ago we had a very, let’s call it opaque understanding of the opposition, and now I’d say it’s even more opaque,” he said in remarks at the Center for Strategic and International Studies. He said he believes the best approach for the U.S. is to continue to try to forge consensus among partner nations in the Middle East and beyond. “I think we should be doing everything we’re doing to, with all of the instruments of power, but the military application of power should be the very last instrument we employ,” he said. “I don’t think at this point I can see a military option that would create an understandable outcome, and until I do, it will be my advice to proceed cautiously.” At the State Department, Kerry said there is a fundamental imbalance in Syria’s civil war. The Assad regime is attacking with tanks, scud missiles and aircraft that the rebels don’t have. And Kerry said Russia, Iran, Hezbollah and al-Qaida-related elements are helping Assad. With al-Qaida and its allies, it’s unclear what support the secretary of state was describing. Al-Qaida in Iraq has clearly backed Syria’s rebels, which has been acknowledged by U.S. and other Western officials, particularly through its relationship with Jabhat al-Nusra — which the U.S. has declared a foreign terrorist organization. State Department spokeswoman Victoria Nuland later said Kerry misspoke, and that concerns over al-Qaida are related to the rebels. Speaking to reporters in Washington, she cautiously addressed the effort by opposition members meeting in Istanbul to form an interim government, saying only that the opposition should maintain unity and represent all Syrians with “the best standards of justice, human rights, democracy.” Nuland also confirmed that Syrian warplanes hit targets across the border in Lebanon on Monday in what she described as a “significant escalation.” “These kinds of violations of sovereignty are absolutely unacceptable,” she said. Syria’s civil war has pushed millions of people from their homes. The U.N. says more than 70,000 people have been killed. ___ Associated Press writers Robert Burns and Donna Cassata contributed to this report.