King Abdullah II vowed Sunday to protect Jordan from the war in Syria, after the United States announced its warplanes and missiles will remain in the kingdom at the end of military exercises.

"Should the world not act and help us adequately address this issue, and should this issue become a threat... then we are capable at any time to take the necessary measures to protect our country and people's interests," said the Jordanian ruler.

"In our handling of the Syrian crisis, the first and ultimate goal has been to protect Jordan's and Jordanians' interests," he told a university graduation ceremony.

The Pentagon on Saturday confirmed that US F-16 fighter jets and Patriot missile interceptors will remain in Jordan after the end of a joint military exercise this month.

The king said Jordan was working with Arab nations as well as the United States, Russia and European countries "to find a political solution that preserves the unity and stability of Syria".

He told graduates at Muta University's military wing in the southern city of Karak that "the crisis in brotherly Syria... imposed on us some harsh realities".

"On the other hand, we are working to secure international financial assistance to cover the costs of hosting the refugees," he added.

A US defence official said this week the Americans would also keep a unit of Marines on amphibious ships off the Red Sea coast after consultations with Jordan.

Washington is concerned about a possible spillover of violence from Syria to its southern neighbour Jordan, a key US ally and one of only two Arab states to have signed a peace treaty with Israel.

Jordan is hosting nearly a half million Syrian refugees, and could serve as a conduit for the military support Washington has said it will give to rebels battling against Syrian President Bashar al-Assad's forces.