The United States is preparing to establish a military presence at an airbase in Damascus to help enable a security pact that Washington is brokering between Syria and Israel, six sources familiar with the matter told Reuters.

The US plans for the presence in the Syrian capital, which have not previously been reported, would be a sign of Syria's strategic realignment with the US following the fall last year of longtime leader Bashar al-Assad, an ally of Iran.

The base sits at the gateway to parts of southern Syria that are expected to make up a demilitarised zone as part of a non-aggression pact between Israel and Syria. That deal is being mediated by US President Donald Trump's administration.

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Reuters spoke to six sources familiar with preparations at the base, including two Western officials and a Syrian defence official, who confirmed the US was planning to use the base to help monitor a potential Israel-Syria agreement.

The Pentagon and Syrian foreign ministry did not immediately respond to requests for comment on the plan. The Syrian presidency and defence ministry did not immediately respond to questions about the plan sent via the Syrian information ministry.

A US administration official said the US was "constantly evaluating our necessary posture in Syria to effectively combat ISIS (Islamic State) and (we) do not comment on locations or possible locations of (where) forces operate."

The official requested that the name and location of the base be removed for operational security reasons. Reuters has agreed to not reveal the exact location.

A Western military official said the Pentagon had accelerated its plans over the last two months with several reconnaissance missions to the base. Those missions concluded the base's long runway was ready for immediate use.

Two Syrian military sources said the technical talks have been focused on the use of the base for logistics, surveillance, refueling and humanitarian operations, while Syria would retain full sovereignty over the facility.

A Syrian defence official said the US had flown to the base in military C-130 transport aircraft to make sure the runway was usable. A security guard at one of the base's entrances told Reuters that American aircraft were landing there as part of "tests".

It was not immediately clear when US military personnel would be dispatched to the base.

Joint Syrian-American presence

The new US plans appear to mirror two other new U.S. military presences in the region monitoring cessation of hostilities agreements: one in Lebanon, which closely watches last year's ceasefire between Lebanese armed group Hezbollah and Israel, and one in Israel that monitors the Trump-era truce between Palestinian military group Hamas and Israel.

The US already has troops stationed in northeastern Syria, as part of a decade-long effort to help a Kurdish-led force there combat Islamic State. In April, the Pentagon said it would halve the number of troops there to 1,000.

Sharaa has said any US troop presence should be agreed with the new Syrian state. Syria is set to imminently join the US-led global anti-ISIS coalition, US and Syrian officials say.

A person familiar with the talks over the base said the move was discussed during a trip by Admiral Brad Cooper, Commander of the US Central Command (CENTCOM), to Damascus on September 12.

A CENTCOM statement at the time said Cooper and US envoy to Syria Thomas Barrack had met Sharaa and thanked him for contributing to the fight against IS in Syria, which it said could help accomplish Trump's "vision of a prosperous Middle East and a stable Syria at peace with itself and its neighbors." The statement did not mention Israel.

The US has been working for months to reach a security pact between Israel and Syria, two longtime foes. It had hoped to announce a deal at the United Nations General Assembly in September but talks hit a last-minute snag.

A Syrian source familiar with the talks told Reuters that Washington was exerting pressure on Syria to reach a deal before the end of the year, and possibly before Sharaa's trip to Washington.