Syrian forces have shot dead unarmed women and children, shelled residential areas and tortured wounded protesters in hospital under orders from the highest level of army and government officials, the United Nations said on Thursday.

Independent U.N. investigators called for perpetrators of such crimes against humanity to face prosecution and said they had drawn up a confidential list of names of commanding officers and officials alleged to be responsible.

The commission received credible and consistent evidence identifying high- and mid-ranking members of the armed forces who ordered their subordinates to shoot at unarmed protesters, kill soldiers who refused to obey such orders, arrest persons without cause, mistreat detained persons and attack civilian neighbourhoods with indiscriminate tanks and machinegun fire, investigators said in a report to the U.N. Human Rights Council.

The commission of inquiry, headed by Brazilian Paulo Pinheiro, found that rebel forces led by the Free Syrian Army had also committed abuses including killings and abductions, although not comparable in scale.

Syrian authorities could not be immediately reached for comment on the commission's latest findings.

But a January 23 letter from its diplomatic mission in Geneva, printed in the report's annex, rejected as totally false allegations contained in the U.N.'s previous report in November that Syrian forces were committing crimes against humanity. The Syrian letter accuses armed terrorist groups of such crimes.

Syrian President Bashar al-Assad's forces bombarded opposition Sunni Muslim districts in the city of Homs for the 20th day on Thursday, activists said, despite international outrage over the reported killing of more than 80 people on Wednesday.

Armoured forces loyal to Assad moved into the rebel district of Baba Amro in Homs on Thursday, opposition sources said.

Syria is on the brink of civil war and deep divisions among world powers complicate the prospects for ending nearly a year of violence sparked by protests against the regime, the three-member U.N. panel said in their latest 72-page report.

The continuation of the crisis carries the risk of radicalising the population, deepening inter-communal tensions and eroding the fabric of society, it warned.

SATELLITE IMAGES CORROBORATE TESTIMONY

The U.N. team was not allowed into Syria but said it had interviewed 369 victims and witnesses. They included people still in Syria whom it contacted by telephone and those who have fled to neighbouring countries which it declined to identify.

Satellite imagery of areas where military and security forces were deployed and related reported violations occurred, corroborated a number of witness accounts, it said.

Thousands of people, mainly civilians but also soldiers and defectors, have been killed during the nearly year-long crackdown, it said.

Army snipers and Shabbiha gunmen posted at strategic points terrorised the population, targeting and killing small children, women and other unarmed civilians. Fragmentation mortar bombs were also fired into densely populated neighbourhoods.

Some 6,399 civilians and 1,680 army defectors were killed in the violence through February 15, according to figures provided by the Violations Documentation Centre, a network of activists in Syria and abroad quoted in the U.N. report.

The level of fighting has increased since November, especially in Homs, Hama and Idlib provinces, with many areas besieged by state forces, according to the report.

On several occasions in January and February 2012, entire families -- children and adults -- were brutally murdered in Homs. On both sides, there is a pattern of abducting people not directly involved in the clashes for the purposes of revenge, ransom or as hostages, the U.N. panel said.

More than 18,000 people were in detention as of February 15, it said, again citing the Violations Documentation Centre.

Security agencies continued to systematically arrest wounded patients in state hospitals and to interrogate them, often using torture, about their supposed participation in opposition demonstrations or armed activities.

The panel, the report said, had documented evidence that sections of Homs Military Hospital and Al Ladikah State Hospital had been transformed into torture centres.

The inquiry, set up by the U.N. Human Rights Council last August, published a preliminary report in November accusing Syrian forces of crimes against humanity including murder, rape and torture. The 47-member forum is to examine the crisis in Syria at a four-week session starting on Monday.

Its other members are Karen Koning AbuZayd of the United States, a former head of the U.N. Relief and Works Agency (UNRWA) helping Palestinian refugees, and Yakin Erturk, a professor from Turkey who has served as U.N. investigator on violence against women.

(Reporting by Stephanie Nebehay; Editing by Mark Heinrich)