Syrian forces loyal to President Bashar Assad fired Scud missiles at rebels in recent days, The New York Times reported Wednesday, citing US officials.
According to one official, Scud missiles were fired from the Damascus area at Free Syrian Army targets in the north of the country, apparently the first such attack inside the country.
It is unclear how many casualties resulted from the Scud attacks.
A senior US official was quoted in the Times as saying that the Obama administration views Assad's use of Scud missiles as a "significant escalation" of the conflict.
“Using Scuds to target tanks or military bases is one thing, using them to target rebels hiding in playgrounds at schools is something else,” the official said.
The Syrian army is using warplanes and heavy artillery to try to halt further advances by rebels, many of them die-hard Islamists. Opposition leaders say they need heavy weapons to sustain the momentum and change the military equation in a conflict that has killed 40,000 people since March 2011.
"Friends of Syria" group meets in Morocco to discuss uprising
On Wednesday, Western and Arab nations sympathetic to Syria's uprising against Assad gave full political recognition to the opposition, reflecting a hardening consensus that the 20-month-old uprising might be nearing a tipping point.
Meeting in the Moroccan city of Marrakech as rebels battled Assad's troops on the outskirts of his Damascus power base - and shortly before the capital was rocked by a huge explosion - the "Friends of Syria" group called on Assad to step aside.
But tensions between the United States and the Syrian opposition coalition surfaced at the same meeting when its leader criticized Washington's designation of the Islamist Jabhat al-Nusra rebel brigade as a terrorist organization.
Hours earlier, US President Barack Obama announced Washington would recognize the coalition of opposition groups, led by Sunni Muslim cleric Mouaz Alkhatib, as Syria's legitimate representative, joining France, Britain, Turkey and Gulf states.
"Participants acknowledge the National Coalition as the legitimate representative of the Syrian people and the umbrella organization under which the Syrian opposition are gathering," said the Marrakech declaration after a meeting attended by 130 ministers and officials.
The gathering brought together Western and Arab nations opposed to Assad, whose family has ruled Syria for 42 years. But it excluded Russia, China and Iran, which have backed Assad or blocked efforts to tighten international pressure on him.
"Bashar Assad has lost legitimacy and should stand aside to allow a sustainable political transition," said the text.
Referring to Western reports suggesting Assad might use chemical and biological weapons, the text said "any use of chemical weapons in Syria would be abhorrent and that this would draw a serious response from the international community".
Qatar reinforced that message after the meeting, urging Assad to take
the "brave decision" to step down, while Britain called on him to end a
war it said he could not win.
Participants announced the creation of a relief fund "to support the Syrian people", calling on states and organizations to make contributions to the fund.
Western officials are due to meet commanders of a newly formed rebel military command in Turkey next week.
Syria's state news agency SANA said Obama's recognition of the political opposition, which coincided with Washington's designation of the radical Islamist Jabhat al-Nusra group, part of the rebel force against Assad, as a terrorist organization "proves American hypocrisy".
Opposition calls on US to reconsider terrorist group designation
Coalition leader Alkhatib urged the United States to reconsider its decision to designate the militant Islamist Jabhat al-Nusra as a terrorist group.
"The decision to consider a party that is fighting the regime as a terrorist party needs to be reviewed," he said.
"We might disagree with some parties and their ideas and their political and ideological vision. But we affirm that all the guns of the rebels are aimed at overthrowing the tyrannical criminal regime."
Alkhatib called on Syria's Alawite minority to launch a campaign of civil disobedience against Assad, an Alawite facing a mainly Sunni Muslim uprising against his rule.
Fighting is moving closer to Assad's residence in the center of Damascus, and early on Wednesday government forces fired artillery and rockets at southwestern suburbs of the capital adjacent to the Mezzeh military airport, activists said.
State television reported an explosion at the gate of the Interior Ministry in Damascus. The ministry is in Kafar Souseh, an area contested by rebels and forces loyal to Assad. A resident reported sirens and shooting after a "huge explosion."
SANA said on Wednesday that "terrorists" detonated two bombs in the Damascus district of Jaramana, killing one person and wounding five, and another two bombs behind the Justice Ministry in Damascus, wounding one person.
In central Syria, an attack on a village killed or injured as many as 200 members of Assad's Alawite minority sect, activists said, but it was unclear who was behind the assault.
The rebels now hold a near continuous arc of territory from the east to the southwest of the capital. With conditions deteriorating, Damascus residents face power and food crises.