By Louis Charbonneau and Dominic Evans
UNITED NATIONS/BEIRUT (Reuters) - Syria's 15-month uprising has grown into a full-scale civil war where President Bashar al-Assad's forces are trying to recapture swathes of urban territory lost to rebels, the U.N. peacekeeping chief said on Tuesday.
"Yes, I think we can say that," U.N. Under-Secretary-General for Peacekeeping Operations Herve Ladsous said when asked if the Syrian crisis could now be characterized as a civil war.
"Clearly what is happening is that the government of Syria lost some large chunks of territory in several cities to the opposition and wants to retake control of these areas," he said.
His remarks, the first time a senior U.N. official has declared Syria's conflict is a civil war, came as the United States said Russia could be sending attack helicopters to Syria.
The International Red Cross said the situation was deteriorating in several parts of Syria simultaneously as fighting intensifies.
Many hundreds of people, including civilians, rebels and members of Assad's army and security forces, have been killed since a ceasefire deal brokered by international mediator Kofi Annan two months ago was supposed to halt the bloodshed.
"Now we have confirmed reports of not only of the use of tanks and artillery but also attack helicopters," Ladsous said in an interview with Reuters and one other reporter. "This is really becoming large scale."
U.S. Secretary of State Hillary Clinton said Washington was worried Russia may be sending Syria attack helicopters and dismissed Moscow's argument that its arms transfers to Syria are unrelated to the conflict there.
"We have confronted the Russians about stopping their continued arms shipments to Syria. They have, from time to time, said that we shouldn't worry - everything they are shipping is unrelated to their (the Syrian government's) actions internally," Clinton said, addressing a forum in Washington.
"That's patently untrue."
United Nations observers overseeing Annan's ceasefire deal were turned back from the town of Haffeh on Tuesday by angry crowds throwing stones and metal rods. As they left, three of their vehicles were fired on, although the monitors were unhurt.
The monitors have been trying since Thursday to visit Haffeh where activists say the army had been battling rebels and the United States has warned of another potential massacre.
The rebel Syrian Free Army said it withdrew from the Sunni Muslim town later on Tuesday under pressure from bombardment by Assad's forces, leaving thousands of civilians without protection.
Activists say Syria's army and pro-Assad militias have committed two mass killings in the last three weeks, in the Houla region and the farming hamlet of Mazraat al-Qubeir. Syrian authorities blamed the killings on "terrorists".
The killings have hardened sectarian divisions between the mostly Sunni Muslim rebels and supporters of Assad - from the country's Alawite minority - in a conflict whose dynamics are also shifting as rebels become increasingly well-armed.
At least 51 civilians were killed across Syria on Tuesday, the Syrian Observatory for Human Rights reported, including 13 who were killed by mortar bombs fired by security forces on a protest in the eastern city of Deir al-Zor. Twelve soldiers also died in clashes with rebels, the Observatory said.
On Tuesday, state television channel Syria TV reported that "terrorist groups" had seized two buses in Homs province and kidnapped all the passengers. It gave no other details.
The International Committee of the Red Cross, the only international agency to deploy aid workers in Syria, said on Tuesday the situation is worsening in several parts of the country simultaneously.
"Our priority is to be able to reach as many people in as many fighting-stricken areas in the shortest delay possible. However, due to the increasingly deteriorating situation, we are unable to answer all the needs at the same time," spokesman Hicham Hassan said.
He said hundreds of people had fled this week from parts of the city of Homs, which activists say has come under the heaviest bombardment since Annan's tattered ceasefire was supposed to come into force on April 12.
Annan's spokesman said on Tuesday he hoped to convene a meeting of an international contact group on Syria soon, saying he was encouraged by broad support for the idea.
But he gave no date for the meeting and said no venue or list of participants has yet been set. The United States and some Western allies have resisted proposals for Iran to take part, accusing Tehran of "stage-managing" Assad's crackdown on the uprising.
The fighting in Haffeh started last Tuesday when rebels clashed with security forces setting up checkpoints to tighten their grip on the town, which lies close to the Mediterranean port city of Latakia as well as the Turkish border.
Twenty-nine civilians, 23 rebels and 68 soldiers have been killed since then and state forces appear intent on seizing control of the rebel-held town. "The question is, at what price?", the Observatory's director, Rami Abdulrahman, said.
Rebels said they were trying to smuggle trapped civilians out of Haffeh, a Sunni Muslim town in the foothills of the coastal mountains which form the heartland of the ruling minority Alawite sect.
They said hundreds of their fighters were facing a tank and helicopter-backed assault by forces loyal to Assad.
"Every few days we manage to open a route to get the wounded out, so some families were able to escape yesterday," said one rebel in Haffeh, who called himself Abdulwudud.
"The situation is dire. Forget the weapons, people need medicine and food. As you know, we're in a state of war in Syria. The army could enter Haffeh in minutes if it wanted but it is trying to crush it instead," said rebel commander Abdulaziz Kanaan, speaking from Turkey.
State television said security forces were continuing "their pursuit of remnants of the terrorists who attacked residents".
A Foreign Ministry statement also reiterated that Syria was committed to Annan's peace plan, but said the agreement gave Damascus the right to prevent attacks on the army and on state institutions.
A United Nations report into children in armed conflict said on Tuesday that children as young as nine had been victims of killing, maiming, arbitrary arrest, torture, sexual violence and use as human shields.
"In almost all recorded cases, children were among the victims of military operations by government forces, including the Syrian Armed Forces, the intelligence forces and the Shabbiha militia," it said.
There were also credible reports of children being recruited by the rebel Free Syrian Army, it said, and in one incident in March the army and Shabbiha placed children aged 8 to 13 in front of the windows of buses carrying military personnel raiding a village.
(Additional reporting by Erika Solomon and Oliver Holmes in Beirut, Stephanie Nebehay and Tom Miles in Geneva, Arshad Mohammed in Washington; Writing by Dominic Evans; Editing by Giles Elgood and Andrew Heavens)