(Adds details, graphic, quote from Samantha Power, paragraphs 5-7)
By Erika Solomon and Michelle Nichols
BEIRUT/UNITED NATIONS, Feb 6 (Reuters) - The United Nations on Thursday welcomed reports that an agreement had been reached to allow the evacuation of civilians from the besieged Syrian city of Homs as well as the delivery of aid, while Washington voiced skepticism about the government's intentions.
The United Nations made clear that it was not a party to the deal and while it was ready to send in aid, it did not yet have the go-ahead from the government and opposition sides in Syria's war to move on the reported agreement.
"The United Nations and humanitarian partners had pre-positioned food, medical and other basic supplies on the outskirts of Homs ready for immediate delivery as soon as the green light was given by the parties for safe passage," U.N. spokesman Farhan Haq said in a statement.
Syria earlier said it reached a deal to allow "innocent" civilians to leave the rebel-held old city of Homs, potentially the first positive result after last week's deadlocked peace talks in Switzerland.
But U.S. Ambassador to the United Nations Samantha Power questioned the sincerity of the Syrian government's intentions.
"Given that the regime, up to this point, has described just about anybody living in opposition territory as a terrorist - and has attacked them as such - you know, we have reason on the basis of history to be very skeptical," she said in New York.
Power added that Washington was "very concerned about anybody who falls into regime hands who comes from a part of the country that has been under opposition control."
The government's announcement came hours after rebels declared a new offensive in the northern province of Aleppo in response to an escalated air assault by government forces trying to recapture territory and drive residents out of opposition-held areas.
Syrian President Bashar al-Assad's forces have used siege tactics to surround and try to starve out rebels holding strategic areas, a technique increasingly copied by rebels as well.
The siege of the old city of Homs has gone on for over a year, and activists say 2,500 people are trapped inside the area struggling with hunger and malnourishment. They represent only a small fraction of besieged Syrians across the country in desperate need of aid.
"The agreement will allow innocent civilians surrounded in the neighborhoods of Old Homs - among them women and children, the wounded and the elderly - an opportunity to leave as soon as the necessary arrangements, in addition to offering them humanitarian aid," said a Syrian Foreign Ministry statement, cited on Syria TV.
"It will also allow in aid to civilians who choose to stay inside the old city."
Delegates from Syria's warring sides met face to face for the first time at the "Geneva 2," peace conference last week, but were unable to agree anything, even a humanitarian deal for Homs that diplomats had hoped could be a relatively easy first step.
A second round of talks is scheduled for next week.
The government statement also did not elaborate on who would be considered an "innocent."
Rebels have rejected similar offers to evacuate women and children in the past because of fear for the fate of any men left behind. Dozens of men disappeared after a similar deal reached in Mouadamiya, west of Damascus.
RIA news agency from Assad's ally Russia quoted an unnamed official at Syria's Defense Ministry saying rebel fighters were keeping civilians in the area as human shields.
"As for civilians, we are not holding them up or refusing them humanitarian aid but the terrorists are the problem," it quoted the source as saying. "Terrorists are claiming that there are only civilians in the Old City who need humanitarian aid. In fact, it's terrorists who are mainly there, including foreign militants, using small groups of civilians held as hostages."
RIA said the evacuation of civilians and entrance of humanitarian aid were due to start early on Friday, but that was no immediately confirmed by the United Nations.
Syria's nearly three-year-old conflict began as peaceful protests against four decades of Assad family rule and devolved into an armed insurgency after a fierce security crackdown.
Now the major Arab state is in a full-scale civil war that has killed more than 130,000 people and forced over 6 million - nearly a third of its population - to flee their homes.
In Aleppo, the Islamic Front, Syria's largest Sunni Islamist rebel alliance, has joined forces with the Nusra Front, an al Qaeda franchise in Syria, to launch an assault dubbed "The just promise approaches," a reference to a Koranic verse about Judgment Day.
Assad's forces recently mounted a series of attacks on the city of Aleppo, once Syria's business hub, using so-called barrel bombs - oil drums or cylinders packed with explosives and metal fragments, dropped out of helicopters.
They are an indiscriminate weapon, that activists say is being used to push civilians out as the army tries to seize the initiative on the long-stalemated Aleppo battlefront.
"All military forces in their bases should head to the front lines, otherwise they will be questioned and held accountable," a joint rebel statement said.
It warned residents near checkpoints and bases held by Syrian government forces to leave within the next 24 hours, saying the areas would be the insurgents' main targets.
Forces loyal to Assad have been making small gains on rebel-held parts of Syria's second city, advances which many opposition sources blame on weeks of rebel infighting that left more than 2,300 combatants dead.
The Islamic Front and some of its secular rebel allies are trying to oust an al Qaeda splinter group, the Islamic State of Iraq and the Levant, with whom they have ideological and territorial disputes.
Since government forces unleashed the barrel-bombing campaign on rebel-held Aleppo last week, residents have been leaving in droves to seek refuge in government-held parts of the divided city.
Others have fled to Turkey, where many have been held up at the border crossing as camps near Aleppo faced overcrowding.
The Syrian Observatory for Human Rights said Nusra Front fighters and the Islamist group Ahrar al-Sham had taken over a large part of Aleppo prison, freeing hundreds of prisoners and killing or wounding dozens of members of the security forces. (Additional reporting by Gabriela Baczynka in Moscow and Louis Charbonneau in New York; editing by Peter Graff, Giles Elgood and G Crosse)