Barack Obama's plan to launch punitive strikes against the regime of Bashar al-Assad received a boost on Wednesday when the Senate foreign relations committee voted in favour of military authorisation.

The influential committee voted by 10 votes to seven in favour of granting the formal military authorisation requested by Barack Obama, paving the way for a full vote on the floor of the Senate early next week in response to alleged chemical weapons use in Syria.

But the committee also voted to accept controversial amendments proposed by hawkish Republican senator John McCain that would explicitly make it a policy of the US to seek to "change the momentum of the battlefield" in ways that would force Assad to negotiate his resignation.

"It is the policy of the United States to change the momentum on the battlefield in Syria so as to create favourable conditions for a negotiated settlement that ends the conflict and leads to a democratic government in Syria," said the second of two amendments proposed by McCain and Democrat Chris Coons.

"A comprehensive US strategy in Syria should aim, as part of a co-ordinated international effort, to degrade the capabilities of the Assad regime to use weapons of mass destruction while upgrading the lethal and non-lethal military capabilities of vetted elements of Syrian opposition forces, including the Free Syrian Army," it added.

The language reflects concerted behind-the-scenes lobbying by McCain, who effectively made Republican Senate support dependent on the White House agreeing to toughen its approach to Syria. The hawkish former presidential candidate has argued for months that the US should more actively take sides in the conflict and use its punitive strikes in reaction to alleged chemical weapons use to achieve broader strategic aims.

Only three Republicans on the committee voted in favour of the amended final resolution – McCain, Jeff Flake and Bob Corker – but if a similar percentage of the full Senate can be persuaded to vote with Democrats next week it would be sufficient to secure half of Obama's desired Congressional mandate.

The White House welcomed the passage of the authorisation. "We commend the Senate for moving swiftly and for working across party lines on behalf of our national security," it said in a statement. "The military action authorised in the resolution would uphold America's national security interests by degrading Assad's chemical weapons capability and deterring the future use of these weapons, even as we pursue a broader strategy of strengthening the opposition to hasten a political transition in Syria."

The extended US mission objective may make it harder to secure sufficient Democrat support in the House though, where a majority of Republicans are also thought to be opposed.

Obama has left open the possibility that he would ignore defeat in the House, insisting that he did not require full congressional authority but was merely seeking extra political backing.

The committee vote was greeted with alarm by some senators, who warned that Congress and the administration were in danger of acting against the wishes of the US people, who are largely unconvinced, according to recent polling. "We should not take lightly that the American people are not with us," said senator Tom Udall of New Mexico. "They are tired of war."

McCain argued it was essential for the White House draft resolution to be toughened up, because otherwise the strikes – in retaliation for Assad's alleged use of chemical weapons – risked having limited strategic impact.

"Without the provision for reversing the momentum on the battlefield the conditions are not created for the departure of Bashar al-Assad," McCain told reporters as he emerged from a classified Senate briefing session before the vote. "There is no policy without that and no strategy."

Meanwhile in the House of Representatives, testimony by Obama administration officials on their desired military strike on Syria exposed deep and persistent divisions amongst legislators, cutting across partisan boundaries.

Several Democratic legislators expressed opposition to a strike sought by a Democratic administration while several Republican legislators deeply opposed to the Obama administration reluctantly backed it during a sometimes-raucous hearing of the House foreign affairs committee.

The scenario posed a challenge for the secretary of state, John Kerry, the secretary of defense, Chuck Hagel, and General Martin Dempsey, the chairman of the joint chiefs of staff, who argued simultaneously that voting against the resolution risked enabling a new avalanche of chemical weapons use and that the US should take only a limited strike to punish dictator Bashar Assad.

Congressman Michael McCaul, the Texas Republican who chairs the homeland security committee, said he had "great pause" that "there is no great outcome here."

"It gets worse and worse because the majority now of these rebel forces – and I say majority now – are radical Islamists," McCaul said.

Even as the Senate committee voted on a resolution that included an urge to change the battlefield momentum, Kerry, Hagel and Dempsey hastened to clarify that any strikes would be limited against military installations and other targets related to Assad's ability to use chemical weapons, not to empower the Syrian opposition.

Congressman Ted Poe, a Texas Republican, asked if there was a possibility the strike could prelude a deeper US military involvement. "I can never drive the risk of escalation to zero, but our limited purpose, our contributions from others and [more] that we will seek, will limit that risk," Dempsey responded.

Hagel, who like Dempsey previously expressed scepticism on Syria, told McCaul, "this is an imperfect situation. There are no good options here. This is complicated. There is no clarity."

Brad Sherman, a California Democrat supportive of the resolution, suggested that if Assad launches a post-strike chemical attack, Congress might authorize a follow-on attack. "We would prefer you have some kind of trigger in there, if he were to come back and use chemical weapons again, we could respond to that," Kerry said.

Gregory Meeks, a New York Democrat, warned that "if we act in a unilateral way, I have huge concerns … [I am] fearful that will isolate the United States." Meeks's concern was shared by Arizona Republican Matt Salmon, who lamented, "we are looking at a near go-it-alone mission."

Kerry disputed the statement, although he did not name many members of the US coalition against Assad. He said that unnamed Arab countries had offered to "cover the cost" of the strikes.

While some legislators of both parties made it clear their minds had been made up about the wisdom or follow of striking Syria, others openly grappled with how to vote.