NATO accused Syrian President Bashar al-Assad's forces of firing Scud missiles that landed near to the Turkish border, in explaining why it was sending anti-missile batteries and troops to the bloc's frontier.
The Syrian government, which finds itself under attack from rebels in the capital Damascus and by a diplomatic alliance of Arab and Western powers, denies firing such long-range, Soviet-built rockets and had no immediate comment on the latest charge.
Admiral James Stavridis, the American who is NATO's military commander, wrote in a blog on Friday: "Over the past few days, a handful of Scud missiles were launched inside Syria, directed by the regime against opposition targets. Several landed fairly close to the Turkish border, which is very worrisome."
It was not clear how close they came. NATO member Turkey, once friendly toward Assad but now among the main allies of the rebels, has complained of occasional bullets and artillery fire, some of which has been fatal, for many months. It sought the installation of missile defences on its border some weeks ago.
"Syria is clearly a chaotic and dangerous situation; but we have an absolute obligation to defend the borders of the alliance from any threat emanating from that troubled state," Stavridis wrote.
Batteries of U.S.-made Patriot missiles, designed to shoot down the likes of the Scuds popularly associated with Iraq's wars under Saddam Hussein in the 1990s, are about to be deployed by the U.S., German and Dutch armies, each of which is sending up to 400 troops to operate and protect the rocket systems.
The Syrian government has accused Western powers of backing what it portrays as a Sunni Islamist "terrorist" attack on it and says Washington and Europe have publicly voiced concerns of late that Assad's forces might resort to chemical weapons solely as a pretext for preparing a possible military intervention.
In contrast to NATO's air campaign in support of Libya's successful revolt last year against Muammar Gaddafi, Western powers have fought shy of intervention in Syria. They have cited the greater size and ethnic and religious complexity of a major Arab state at the heart of the Middle East - but have also lacked U.N. approval due to Russia's support for Assad.
Moscow reacted angrily on Friday to the way U.S. officials seized on comments by a top Kremlin envoy for the Middle East as evidence that Russia was giving up on Assad. Comments by Mikhail Bogdanov on Thursday in which he conceded Assad might be ousted did not reflect a change in policy, the Foreign Ministry said.
Assad's diplomatic isolation remains acute, however, as Arab and Western powers this week recognised a new, united coalition of opposition groups as Syria's legitimate leadership. Large parts of the country are no longer under the government's control and fighting has been raging around Damascus itself.
European Union leaders who met in Brussels on Friday said all options were on the table to support the Syrian opposition, raising the possibility that non-lethal military equipment or even arms could eventually be supplied.
In their strongest statement of support for the Syrian opposition since the uprising began 20 months ago, EU leaders instructed their foreign ministers to assess all possibilities to increase the pressure on Assad.
With rebels edging into the capital, a senior NATO official said that Assad is likely to fall and the Western military alliance should make plans to protect against the threat of his chemical arsenal falling into the wrong hands.
Desperation for food is growing in parts of Syria and residents of the northern city of Aleppo say fist fights and dashes across the civil war front lines have become part of the daily struggle to secure a loaf of bread.
"I went out yesterday and could not get any bread. If only the problem was just lack of food - there is also a huge shortage of fuel, which the bakeries need to run," said Ahmed, a resident of the battle-scarred Salaheddine district.
He said people get into fist fights over flour and rebels regularly have to break up fights by firing into the air.
The World Food Programme (WFP) says as many as a million people may go hungry this winter, as worsening security conditions make it harder to reach conflict zones.
Forty thousand people have now been killed in the most enduring and destructive of the Arab revolts. The government severely limits press and humanitarian access to the country.
U.N. humanitarian chief Valerie Amos said on Friday the United Nations is committed to maintaining aid operations in Syria.
"Nothing off the table"
At the EU summit, Britain's David Cameron pushed for an early review of the arms embargo against Syria to possibly open the way to supply equipment to rebels in the coming months. Germany and others were more reluctant and blocked any quick move. But there was widespread agreement that whatever action can be taken under current legislation should be pursued, and the arms embargo would still be reviewed at a later stage.
"I want a very clear message to go to President Assad that nothing is off the table," Cameron told reporters at the end of a two-day summit. "I want us to work with the opposition ... so that we can see the speediest possible transition in Syria.
"There is no single simple answer, but inaction and indifference are not options."
Among factors holding Western powers back from arming the rebels is the presence in their ranks of anti-Western Islamist radicals. Following a U.S. decision this week to blacklist one such group, Jabhat al-Nusra, a "terrorist" group, thousands of Syrians demonstrated on Friday against ostracising the movement.
The latest, weekly Friday protests in rebel-held areas were held under the slogan: "The only terrorism in Syria is Assad's".
Inspired by Arab uprisings across the region, Syrian protesters were met with gunfire by Assad's security forces in March 2011. Armed revolt overtook the movement, which has become increasingly sectarian - waged by majority Sunni Muslims against forces loyal to Assad, who is from the minority Alawite sect, an offshoot of the Shi'ite Islam practised in Assad's ally Iran.
A video posted on the Internet showed dozens of Sunni rebels dressed in camouflage gear congratulating and kissing each other outside a burning Shi'ite shrine.
A fighter holding a rifle said the group was destroying the "dens of the Shi'ites". Reuters could not independently verify the video, which was posted on YouTube on Wednesday and purports to be filmed in the northern town of Jisr al-Shughur.