GENEVA (AP) — A showdown between government troops and opposition forces in Syria’s largest city, Aleppo, is imminent, the U.N.’s human rights office said Friday, as the Red Cross reported it is pulling some of its foreign staff from Damascus out of concern for the safety of its workers.

The country’s chaos has spread to Syria’s biggest cities in some of the most widespread and sustained violence the two areas have seen in more than 17 months of conflict. Rebels have been locked in fierce fighting with government troops in Aleppo for six days and are bracing for an attack amid reports that the regime is massing reinforcements to retake the embattled city of 3 million.

U.N. High Commissioner for Human Rights Navi Pillay said unconfirmed reports are coming out of the capital, Damascus, of extra-judicial killings and shootings of civilians during fighting in the city’s suburbs. Expressing deep alarm at the situation, Pillay said the report “bodes ill for the people of that city (Aleppo).”

Pillay said she believes President Bashar Assad’s regime and opposition forces are both committing crimes against humanity and war crimes.

“And it goes without saying, that the increasing use of heavy weapons, tanks, attack helicopters and — reportedly — even jet fighters in urban areas has already caused many civilian casualties and is putting many more at grave risk,” she said in a statement read aloud to reporters by her spokesman Rupert Colville.

Pillay said there have been clashes in Homs and Deir el-Zour and there is a pattern of government forces trying to clear areas it says are occupied by opposition forces. She also expressed concern about reports of killings of unarmed prisoners and use of excessive force by authorities reacting to unrest in two prisons in Aleppo and Homs.

There has been an accompanying rise in the number of reports of opposition fighters torturing or executing prisoners, she said.

The U.N.’s outgoing chief observer in Syria said Friday it’s just a matter of time before President Bashar Assad’s regime crumbles, but that the violence of the civil war could worsen if Syria uses the full force of its military.

Maj. Gen. Robert Mood, whose three-month mission in Damascus ended last week, also said the situation in Syria is likely to remain unstable even if Assad’s government steps down.

“It’s impossible to imagine a future in Syria where the current people in power remain in power. So in that view, it’s just a matter of time before this regime collapses. And that is how it’s supposed to be,” Mood told a news conference in Oslo. That could happen, he said, if more members of the military simultaneously leave the ranks of the government to join the opposition.

But Mood also said that if Assad’s regime collapses, it might not be the end of Syria’s many problems. “That might not be the start of a dialogue. That could easily be the start of a situation that is way worse,” he said.

A senior U.N. diplomat close to the mediation effort of international envoy Kofi Annan said they are “watching the situation in Aleppo with great concern.”

“The ground is shifting. We use words like ‘It’s fluid’ — and it certainly is … It has been a roller-coaster ride,” the diplomat said, while speaking on condition of anonymity because of the delicacy of the negotiations among world powers on the U.N. Security Council. “While we are trying to apportion blame (for the diplomatic standoff), people are dying. Kids are being slaughtered.”

The International Committee of the Red Cross said Friday it is temporarily moving some of its foreign staff from Damascus to neighboring Lebanon. A Red Cross spokesman in Geneva, Hicham Hassan, said the move was prompted by security concerns but that a core team of about 50 staff would remain.

Hassan also told The Associated Press on Friday that the Syrian Arab Red Crescent was suspending some of its operations in Aleppo due to heavy fighting but that the Red Cross hopes to bring its staff back into the country.

“It’s due to the deteriorating security situation in Damascus,” he said. “This in no way means we are suspending our activities, especially at a time when needs are growing. … They will go back to Damascus at the appropriate time.”

The Red Cross and Syrian Arab Red Crescent have helped 600,000 people in Syria since the outbreak of fighting.

About half of the 1.5 million people in need of humanitarian aid in Syria are children and adolescents, UNICEF spokesman Patrick McCormick told reporters Friday in Geneva. The U.N. children’s agency, with the help of Syrian Arab Red Crescent and other groups, he said, has delivered food baskets, high protein biscuits and other items to 190,000 people, including 145,000 children, since January.

McCormick said they also have vaccinated a half-million Syrian children again polio and tuberculosis, but many of the children in refugee families are hungry and have lost a year of schooling and need medical and psychological care.

Turkey’s state-run agency said a Syrian legislator from Aleppo has fled to Turkey and also warned that Syria was preparing for a massive offensive on cities where rebels are fighting government forces. The Anadolu agency said Friday that Ikhlas Badawi has defected in protest of the Syrian regime’s “violence against the people.”

She would be the first member of Syria’s parliament to defect from the parliament that was elected in May.

In January, Legislator Imad Ghalioun left the country to join the opposition, saying Syria was suffering sweeping human rights violations. He was from the city of Homs that was being subjected to a massive regime attack at the time.

On Friday, the Britain-based Syrian Observatory for Human Rights said troops bombarded the neighborhood of Fardous killing at least four people. It added that Sunni cleric Abdul-Latif al-Shami was kidnapped and killed in Aleppo. It gave no further details, although some activists said al-Shami is a government supporter.

Mohammed Saeed, an Aleppo-based activist, said helicopters were firingwith heavy machine-guns rebel-held areas east and west of the city on Friday. He added that army reinforcements arrived in the city on Thursday and a major attack is expected any time.

Rami Abdul-Rahman, who heads the Observatory, said army reinforcements took positions around Aleppo. “I expect the attack to begin today,” he said.

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Mroue reported from Beirut. Frank Jordans contributed from Berlin.

Copyright 2012 The Associated Press.