The United States will deploy Patriot missiles and F-16 fighter jets to Jordan for military exercises this month - and may consider keeping them there longer, in consultation with Jordan, U.S. officials said on Tuesday.
The disclosure drew swift condemnation from Moscow, which accused the West of sending weapons to fuel Syria's civil war.
The Eager Lion exercises - held annually with a theme of irregular warfare - will include more than 8,000 service members from about 19 countries, one U.S. official said, adding the drills would run from June 9 to 20.
"These annual exercises will increase the preparedness of the Jordanian army. This year we are in need of more advanced weapons," Jordanian Minister of Information Mohammad al-Momani told Reuters.
If left in Jordan, Patriot missiles could be used to protect the country - a Sunni Muslim U.S. ally - against any possible missile attack as the Syrian war threatens to widen into a more regional, sectarian conflict.
There was no official statement suggesting the Patriots or the fighter jets would be withdrawn when the exercises are over and U.S. officials left open the possibility they could remain in place.
"We will consider extending the deployment of assets associated with Eager Lion in consultation of the government of Jordan," a second official said.
Jordan is one of the Arab countries that backs the opposition to Syrian President Bashar Assad, who is fighting insurgents in a two-year-old civil war that has killed 80,000 people.
President Barack Obama has been reluctant to become involved in the Syrian conflict despite having called for Assad to resign and hinting at U.S. military action if the Syrian government crossed a "red line" by using chemical weapon.
Beyond any deployment of Patriot missiles, the United States has other military and political options on Syria, such as arming the rebels or setting up a no-fly zone for Assad's air force.
The decision to send Patriot missiles to Jordan is particularly controversial for Russia, Assad's main global ally, which believes the missiles could be used by the United States and its allies to impose a no-fly zone over Syria, heralding the first direct Western military intervention in the conflict.
"We have more than once stated our opinion on this - foreign weapons are being pumped into an explosive region," Russian Foreign Ministry spokesman Alexander Lukashevich said in a statement.
"This is happening very close to Syria, where for more than two years the flames are burning of a devastating conflict that Russia and its American partners are trying to stop by proposing to hold an international peace conference as soon as possible."
Moscow complained vociferously last year when the United States, Germany and the Netherlands deployed Patriots on Syria's northern border in Turkey, a NATO ally. NATO said the Patriots were sent there as a precaution in case missiles were fired over the border from Syria.
Moscow said that decision was a factor in its decision to go ahead with plans to send its own anti-aircraft system, the S-300, to Assad's government.
The Russian system has not yet been deployed but Moscow has said it would fulfill the delivery contract.
Assad's air power is one of his main advantages against the rebels, who are relatively lightly armed with weapons they receive from Saudi Arabia and Qatar.
Despite their differences over supplying weapons, the United States and Russia have jointly called for a peace conference on Syria later this month, the first attempt in a year by the powers supporting the opposing sides in the civil war to find a diplomatic solution.
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