WASHINGTON (REUTERS) - The United States will "take firm and appropriate measures" in response to Syrian government violations in a so-called de-escalation zone in the southwest of the country, the State Department said in a statement on Thursday (June 14).

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

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" of the country.

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

As a member of the UN 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 in its statement.

The State Department also said that the United States would release US$6.6 million (S$8.9 million) to the Syrian Civil Defence fund, commonly known as the White Helmets, and the UN International, Impartial and Independent Mechanism, which collects and analyses evidence of violations of international human rights laws.

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