U.S. Secretary of State Hillary Clinton is hopeful NATO allies will agree this week on stationing Patriot missiles in Turkey to defend against possible Syrian attacks, senior U.S. officials said.
She also reiterated a warning against any attempt by the Syrian government to use its chemical weapons stockpile against the rebels, calling this was a "red line" that would prompt U.S. action.
The 28 NATO allies will meet in Brussels on Tuesday and Wednesday.
Turkey, which has formally asked NATO to help it bolster its air defenses, is a big supporter of rebels fighting in a 20-month-old uprising to oust Syrian President Bashar al-Assad.
It has repeatedly scrambled jets along the countries' joint border and responded in kind when shells from the conflict have crashed inside Turkey, underlining fears Syria's civil war could spread to destabilize the region.
"We're all positively considering (the request)," a senior State Department official said, briefing reporters en route to Europe, where Clinton started a regional trip with a stop in Prague.
"(We) are hopeful that NATO will be in a position to respond positively ... and that the three contributing countries that are being considered - the United States, Germany and the Netherlands - will be in a position to also contribute."
The State Department official said he did not expect final details this week on the numbers of missiles that would be deployed, or where or for how long, as site surveys were still going on.
He also said it would probably be "at least a matter of weeks" before deployment, as national decisions still had to be made and surveys completed and agreed.
Russia, which will join the NATO meeting, has been at odds with the alliance over how to end the Syrian conflict. Russia has vetoed U.N. resolutions aimed at pressuring Assad to step down and Moscow's foreign minister, Sergei Lavrov, is expected to raise concerns over the missile deployment plans.
The State Department official said the deployment would not be part of "an inexorable move towards a no-fly zone" over Syria of the sort NATO mounted to defend anti-government rebels in Libya who toppled Muammar Gaddafi last year.
The official also rejected the idea that deployment of Patriots in Turkey would create a de-facto safe haven in Syria's border area with Turkey, as the missiles would be used to defend against planes or missiles that crossed into Turkish air space.
NO-FLY ZONE "NOT ON AGENDA"
The possibility of establishing a no-fly zone is expected to be discussed at a meeting of the Friends of Syria group in Marrakech next week.
"We're always prepared - and the Secretary has made that clear - to look at ways in which we can help the people of Syria," the official said. "But ...a no-fly zone is not on the agenda of any NATO talks this week."
France, Britain, Turkey and Gulf Arab states have already recognized a rebel coalition as the legitimate representative of the Syrian people. The United States has been more cautious and not offered full recognition, or arms the rebels are seeking.
Clinton, at a news conference with Czech counterpart Karel Schwarzenberg, again warned Syria against resorting to chemical weapons against the insurgents.
"I am not going to telegraph any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people, but suffice to say, we are certainly planning to take action if that eventuality were to occur," she said.
"So we once again issue a very strong warning to the Assad regime that their behavior is reprehensible, their actions against their own people have been tragic, but there is no doubt that there is a line between even the horrors they have already inflicted on the Syrian people and moving to what would be an internationally condemned step of utilizing their chemical weapons."
Clinton will also hold talks in Brussels on Monday with Pakistani Foreign Minister Hina Rabbani Khar and Islamabad's military chief General Ashfaq Kayani.
U.S. officials say she will seek to encourage what Washington sees as an improving mood between Pakistan and its neighbor Afghanistan, where U.S.-led forces are seeking to extricate themselves from a long and bloody war.
A second U.S. official pointed to better signs since Washington's ties with Islamabad hit a low in 2011 after U.S. forces killed al Qaeda leader Osama bin Laden while he was hiding in the Pakistani city of Abbottabad.
He noted exchanges of high-level visits between Pakistan and Afghanistan, where NATO still has a large multinational force that is due to end its combat operations in 2014, and Pakistan's release of Taliban prisoners as requested by the Afghans to help smooth the way for peace talks with the Islamists.
"I think the Pakistanis are actually pressing forward (on improving relations) because, like a lot of people in the region, they recognize that 2014 is not so far away," he said.
As well as discussions on security, the United States had made efforts to expand the economic relationship with Pakistan and this was an issue expected to come up in Monday's talks.
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