Senator Edward J. Markey urged President Trumpon Saturday to outline a comprehensive strategy on how the United States will deal with Syria and to seek authorization from Congress before escalating the conflict further.

“It’s one thing to have a tactic, it’s another thing to have a strategy,’’ said Markey at a press conference at the John F. Kennedy Federal Building. “Donald Trump last week was in favor of letting [Syrian President Bashar] Assad stay in power; this week he’s in favor of removing Assad. That is political whiplash of the highest order.’’

His points were echoed by Representative Seth Moulton, who discussed Syria with reporters after a town hall event in Lynn. The assault Thursday on an air base in central Syria was retaliation for a deadly attack that US officials believe Syrian government aircraft launched using the nerve agent sarin mixed with chlorine gas.

“The point is not to have a military plan, but to have a political plan that underlies any military action that we take,’’ said Moulton, a combat veteran who serves on the House Armed Services Committee. “I don’t think that those political goals are clear at all in Syria today.’’

The comments from Markey and Moulton came as the White House released a letter sent to leaders of both houses of Congress in which Trump said he ‘‘acted in the vital national security and foreign policy interests of the United States’’ in ordering the attacks.

The letter, Trump said, was intended in part to keep Congress informed of his actions, “consistent with the War Powers Resolution.’’

That law says a president may order military action in a conflict only with congressional approval or if the US is attacked.

But it also requires presidents to halt actions after 60 days if they don’t have approval, a provision that has been sometimes used to justify brief operations or one-time interventions.

At Moulton’s town hall event, Javier Lira, a Navy veteran from Peabody, asked whether Congress is taking action to amend laws authorizing the use of force.

Moulton said the issue needs to be reexamined and called on Congress to update a 2001 law that gave President George W. Bush the power to attack those responsible for the Sept. 11 terrorist attacks and those who harbored them.

“While I think [Trump] was within legal bounds to take the initial strike that he took, any further action will require the consultation with Congress,’’ Moulton told reporters. “We’re in a very different place in this war than where we were right after 9/11, and it’s time for Congress to do its job’’

In statements issued Friday, other members of the state’s congressional delegation asked Trump to consult with Congress before taking further action in Syria.

“Given the many questions and complexities of the situation in Syria, the White House must divulge its overarching strategy to the American people and to Congress,’’ said Representative Niki Tsongas, who also serves on the House Armed Services Committee.

Representative Michael E. Capuano, who was one of four members of Congress who sued President Barack Obama to force him to seek approval for bombing Libya, urged his colleagues to debate even the most “politically sensitive’’ aspects of military action in Syria.

Only Congress, Capuano said, can declare war, and a missile strike against a sovereign state “no matter how horrendous its government’’ is an act of war.

The US attack has also strained relations with Russia, which backs Assad and brokered a 2013 deal to rid Syria of chemical weapons.

Russia condemned Trump’s use of force and announced that it was suspending an agreement to share information about air operations over Syria that was devised to avoid accidental conflict.

On Friday, US officials said the Pentagon was investigating whether Russia aided in the chemical attack that killed scores in Idlib province.

“The risks of escalation and particularly with Russia have never been greater than they are today,’’ Moulton said. “It’s also concerning frankly that overshadowing all of this is we still don’t understand the relationship between the Trump administration and the Russian government and the Russian intelligence services.’’

Markey said Trump must hold Russian officials to their commitment to remove chemical weapons from Syria. Russia must also enforce the cease-fire it reached with Iran and Turkey, he said.

The intervention in Syria was unpopular among some who attended Moulton’s event at North Shore Community College.

Jeanne Deffley, a retired nursing supervisor from Newburyport, said she is suspicious of Trump’s motives.

“I think it’s a political ploy, perhaps in collusion once again with Russia,’’ she said. “This is a dangerous thing to do.’’

Lira, who asked Moulton about congressional authorization for the use of force, said the goals of the attack are unclear. “There’s no agreement on policy,’’ he said.

Dan Liebert, who lives in Salem, said the options for Syria “are really quite bad.’’

“If they actually used the chemical weapons, yes, there should have been consequences,’’ he said. “What now? I don’t know.’’

Material from the Associated Press was used in this report. Evan Allen can be reached at evan.allen@globe.com. Follow her on Twitter @evanmallen. Laura Crimaldi can be reached at laura.crimaldi@globe.com. Follow her on Twitter @lauracrimaldi.