AMMAN (Reuters) - Jordan is confidant that Russia will not allow President Bashar al-Assad’s government to wreck a ceasefire in southern Syria, a Jordanian official said on Monday, days after Washington said it feared that the Syrian government plans an offensive there.

With Syrian government forces now in their strongest position since the early months of the seven-year civil war, the southwest is one of the few parts of the country still in the hands of enemies of President Bashar al-Assad.

The area is covered by a ceasefire sponsored by the United States, Russia and Jordan, the result of the first Syrian peacemaking diplomatic effort of President Donald Trump’s administration.

The United States says it is concerned that Assad’s government is planning to jettison the truce and launch a major offensive in the area to fulfill Assad’s goal of re-asserting control over every inch of his country. Washington warned Syria on Friday it would take “firm and appropriate measures” in response to any ceasefire violations.

But the senior Jordanian official, who requested anonymity, told Reuters that Russia, Washington and Jordan all still saw “eye to eye” on the need to preserve the ceasefire.

“The de-escalation zone has produced the ceasefire that has held best in all of Syria. The parties to the agreement are all committed to preserving it,” the official said.

The three countries that sponsored the ceasefire were holding discussions to ensure it did not collapse, the Jordanian official said.

Jordan has long been worried about the presence of Iranian-backed militias along its border, as has Israel.

Both Israel and Jordan have been seeking understandings with Moscow to push these Shi’ite militias away from the area. Israel has stepped up its military strikes on suspected Iranian targets across Syria in recent weeks.

“We are closely watching developments and will protect our national security,” the Jordanian official said of the “fluid” situation in southern Syria.

Assad’s government has recaptured all rebel-held territory near the capital this year. Areas in the southwest and the northwest are the only large parts of the country still in control of rebels seeking to overthrow him.

(This story has been refiled to correct typo in headline.)