(Reuters) - The United States will “take firm and appropriate measures” in response to Syrian government violations in a designated de-escalation zone in the country’s southwest, the State Department said on Thursday.

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After seven years of conflict, the southwest of Syria, bordering Jordan and the Israeli-occupied Golan Heights, is one of the parts of the country still outside the state’s control.

President Bashar al-Assad’s forces have recovered swathes of territory from rebels with the help of Russian air power and Iran-backed militias, and he has repeatedly pledged to take back “every inch”.

Experts have investigated reported use of chemical weapons by Assad’s government. More than 500,000 people have been killed in the fighting, which has drawn in global powers and neighbouring states.

Assad said on Wednesday his government was still pursuing a political solution for Syria’s rebel-held southwest, but would use military force if the effort failed.

“We are giving the political process a chance,” he told Iranian channel al-Alam News in an interview published in an English translation by Syrian state news agency SANA.

“If that doesn’t succeed, we have no other option but to liberate it by force.”

Since last year, a “de-escalation” deal brokered by Jordan, Russia and the United States has contained fighting in the southwest.

The state department said any Syrian government military actions against that zone risked broadening the conflict.

“We affirm again that the United States will take firm and appropriate measures in response to Syrian government violations in this area,” it said in a statement on Thursday.

An underpinning ceasefire arrangement and the de-escalation deal are intended to save lives and create conditions for the displaced to return home safely, it added.

“The ceasefire must continue to be enforced and respected,” it said.

The statement comes after Syrian jets struck rebel-held towns in the country’s south in March, rebels and residents said, the first aerial attacks on the area since the pact declaring it a “de-escalation zone.”

As a member of the U.N. Security Council, Russia “is duly responsible” to “use its diplomatic and military influence over the Syrian government to stop attacks and compel the government to cease further military offensives,” the state department said.

Early on Thursday, the State Department said the United States would release $6.6 million to the Syrian Civil Defence fund, commonly known as the White Helmets, and the U.N. International, Impartial and Independent Mechanism, which collects and analyses evidence of violations of international human rights laws.