President Barack Obama gained ground Tuesday with his drive for congressional backing of a military strike against Syria, winning critical support from the top Republican in Washington while administration officials agreed to explicitly rule out the use of U.S. combat troops in retaliation for a chemical weapons attack.

The Republican speaker of the House of Representatives, John Boehner, said taking action is something “the United States as a country needs to do.”

Boehner emerged from a meeting at the White House and said the United States has “enemies around the world that need to understand that we're not going to tolerate this type of behavior.”

Obama on Saturday unexpectedly stepped back from ordering a military strike under his own authority and announced Saturday he would seek congressional approval.

The president urged Congress to hold a prompt vote once it returns from holiday next week.

He also tried to assure the public that involvement in Syria will be a “limited, proportional step.”

“This is not Iraq, and this is not Afghanistan,” Obama said.

He met with top lawmakers hours before he leaves on a three-day trip to Europe, with a visit to Sweden and a G-20 summit in Russia.

Lawmakers in both the Republican and Democratic parties called for changes in the president's requested legislation, rewriting it to restrict the type and duration of any military action that would be authorized, possibly including a ban on U.S. combat forces on the ground.

A new resolution was written Tuesday by Sen. Bob Menendez, the Democratic chairman of the Senate Foreign Relations Committee, and Sen. Bob Corker, the top Republican on the panel. It could get a vote by the committee Wednesday.

At a hearing of the committee, Secretary of State John Kerry said it would not be a problem to include language establishing “zero capacity for American troops on the ground.”

Kerry, one of three senior officials to make the case for military intervention at the hearing, had said earlier that he'd prefer not to have such language, hypothesizing the potential need for sending ground troops “in the event Syria imploded” or to prevent its chemical weapons cache from falling into the hands of a terrorist organization.

“President Obama is not asking America to go to war,” Kerry said. He added, “This is not the time to be spectators to slaughter.”

The U.S. says it has proof that the Assad regime is behind sarin gas attacks that Washington claims killed at least 1,429 people, including more than 400 children. The Britain-based Syrian Observatory for Human Rights, which collects information from a network of anti-regime activists, says it has so far only been able to confirm 502 dead.

The Obama administration argues that the United States must exert global leadership in retaliating for what apparently was the deadliest use of chemical weapons anywhere over the past 25 years.

Boehner's support is key, but opposition Republicans in Congress do not speak with one voice.

And after a decade of war in Iraq and Afghanistan, polls show most Americans opposed to any new military action overseas.

Some lawmakers say Obama still hasn't presented good evidence that Assad's forces were responsible for the Aug. 21 attack. Others say he hasn't explained why intervening is in America's interest.

Those questions come a decade after the Bush administration badly misrepresented the case that Iraqi President Saddam Hussein possessed weapons of mass destruction.

U.N. Secretary-General Ban Ki-moon urged caution. He said any punitive action against Syria could unleash more turmoil and bloodshed, and he advised that such strikes would be legal only in self-defense under the U.N. charter or if approved by the organization's Security Council.

Russia and China have repeatedly used their veto power in the council to block action against Assad.

Among major allies, only France has publicly offered to join the United States in a strike, although President Francois Hollande says he will await Congress' decision. The British House of Commons rejected a military strike last week.

In the Middle East, Israel and the U.S. conducted a joint missile test over the Mediterranean in a display of military might in the region.

House Republican Majority Leader Eric Cantor also said he would support Obama's call for military action against Syria.

Obama also won conditional support Monday from two of his fiercest foreign policy critics, Republican Sens. John McCain and Lindsey Graham.

The administration argues that the alleged sarin gas attack last month violated not only the international standard against using such weapons but also Obama's “red line,” set more than a year ago, that such WMD use wouldn't be tolerated.

Obama said he believes he has the authority to undertake limited military action without congressional backing, but he has stated that the United States will be stronger if lawmakers grant their support.

Neither Obama nor his aides, however, has been willing to state what options would be left to him should Congress reject his call. (AP)