The U.S. "will need to act more aggressively" if the Syrian regime or Russia breaks a newly implemented ceasefire, according to a foreign policy expert on Syria.

Ilan Goldenberg, a senior fellow and director of the Middle East Security Program at the Center for a New American Security, wrote in an article Monday that the ceasefire deal's biggest weakness is the lack of measures to punish violations. Goldenberg is also an adviser to Dem nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE on Middle East issues, in particular Israel and Palestine.

"If the regime and its allies break the cessation of hostilities, the United States will need to act more aggressively to empower acceptable elements of the Syrian opposition and develop credible threats of limited military force to deter Assad regime and Russian bombing of civilians in areas controlled by acceptable American supported opposition groups," wrote Goldenberg and co-author Nicholas Heras, a Bacevich fellow at CNAS.

Secretary of State John Kerry John Forbes KerryThe Memo: Warning signs flash for Trump on debates Divided country, divided church TV ads favored Biden 2-1 in past month MORE and Russian Foreign Minister Sergei Lavrov reached an agreement over the weekend for a short-term ceasefire in Syria. The deal went into effect on Monday.

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The agreement calls for a seven-day ceasefire, beginning sundown on Monday. If the ceasefire holds for that period, Russia and the U.S. would begin talks on military cooperation against terrorist groups in Syria.

Goldenberg and Heras also expressed skepticism over whether the U.S. and Russia could cooperate on targeting terrorists.

The two countries appear divided on which groups they would target.

Russia, which intervened in Syria last September on behalf of the Syrian regime, considers opponents of Bashar al-Assad to be "terrorists," while the U.S. wants to target the Islamic State in Iraq and Syria, and al-Qaeda affiliates

One group, al Nusra, officially ended its formal affiliation with al Qaeda in an attempt to be treated as a mainstream opposition group, though some experts say the move was only intended to avoid being targeted by the U.S.

Goldenberg and Heras expressed doubt over whether al Nusra could be separated from other U.S.-backed opposition groups.

"The theory behind this deal is that a renewed cessation of hostilities will protect the civilian population in opposition-controlled areas, which will save lives and provide the socio-political space for non-extremist Syrian rebel groups to take charge of the situation in their areas and marginalize extremists," they wrote.

"One challenge is that JFS has become more 'mainstream' since it formally disassociated from al-Qaeda. Because it is an integral part of armed opposition campaigns in northern Syria, it will not be easily shunned by other elements within the Syrian armed opposition."

Despite some of Clinton's advisers pushing for the U.S. to threaten military force against the Assad regime, the presidential candidate herself promised last week that she would never commit "ground troops" to Syria.

The U.S. already has about 300 troops in the country, advising Kurdish and Arab rebel fighters, and more than 5,000 in Iraq, supporting Iraqi forces battling ISIS.

Rep. Eliot L. Engel (D-N.Y.), ranking member of the House Committee on Foreign Affairs, also expressed skepticism over the agreement, and called for "Plan B" if Russia or Syria broke the ceasefire.

"I welcome news of an understanding between the United States and Russia, though I remain wary of Russian intentions in Syria. The governments of Russia and Syria do not have a strong history of abiding by ceasefires or protecting civilians," he said.

"If these negotiations are successful and can help the people of Syria reach that goal, all the better. However, if this understanding fails, it is time to move on to Plan B," Engel said.

The lawmaker has sponsored legislation that would impose sanctions on supporters of the Assad regime, as well as initiate investigations into Syrian regime war crimes.

This story was updated on Sept. 13 at 10:20 a.m.