US Secretary of State John Kerry on Wednesday discussed a $2.1 billion deal with Omani leaders to supply an American-made air-defence system to the Arab state facing Iran in the Gulf.

Kerry told Oman's Defence Minister Sayyid Badr al-Busaidi that the United States was "excited" about the deal and "very grateful for your confidence in Raytheon," manufacturers of the defence system.

The details of the contract are still being worked through, and a letter of intent is to be signed "soon, but they are still finalising technical details," a State Department spokeswoman said, asking not to be named.

US officials said earlier that Oman had decided in January to buy a ground-based air defence system produced by US giant Raytheon, which is based in Kerry's home state of Massachusetts.

The new top US diplomat had strongly backed the proposed deal before he took office in February.

"Part of the goal of this is to push US commercial interests, to demonstrate to Oman that these are important to this administration," another US official had said before Kerry arrived in Oman on Tuesday.

"We have initiated the talks" about the system, Busaidi said as he met Kerry in the defence ministry in Muscat on Wednesday for talks also attended by Raytheon senior director Ken Gordon.

The Omanis believed it was "the best and most effective system that is out there," Busaidi said

"Talks are underway at this point, and we are at the technical stages but we are hoping to move to the final discussion and sign a contract," he added.

"We are in need of this defence system, which also pertains to the defence strategy of the other countries of the Gulf Cooperation Council."

US officials had earlier said that it was expected that the letter of intent would be signed in Muscat on Wednesday.

Kerry also met with Oman's ruler Sultan Qaboos on Tueday and told US embassy staffers at a meeting the next day that he had appreciated his insight into moves to bring Israel and the Palestinians back to the negotiating table.

After his brief visit to Oman, Kerry was leaving to attend a key meeting of the "Friends of Syria" group in Amman later Wednesday.

He was expected to meet with Jordan's King Abdullah as well as ministers from the core 11 nations of the Friends of Syria as they seek to finalise plans for a peace conference to end the conflict now in its third year.

He will travel to Israel for talks Thursday and Friday with Israeli and Palestinian leaders.