Updated

The Latest on the conflict in Syria (all times local):

8:45 p.m.

The United Nations says the Syrian government has "systematically rejected" repeated requests to visit prisons and detention centers where thousands of prisoners are believed to be subjected to cruel treatment, including torture.

U.N. spokesman Stephane Dujarric told reporters Tuesday that the United Nations cannot independently verify U.S. reports of a crematorium in the Saydnaya military prison, where the U.S. says the bodies of those executed are burned.

But Dujarric said "various U.N. entities have regularly documented and reported on human rights violations in Syria, including torture in the context of detention."

"We are extremely concerned that thousands of civilians continue to be held in government detention facilities and have grounds to believe that they are systematically subjected to cruel, inhuman, and degrading treatment including torture and sexual violence," Dujarric said.

President Bashar Assad's government has denied the U.S. allegations.

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6:30 p.m.

A senior Israeli official has called for President Bashar Assad to be assassinated over alleged atrocities in the Syrian civil war.

Housing Minister Yoav Galant, a former general, said Tuesday: "As far as I am concerned it is time to assassinate Assad."

He said "when we finish with the tail of the snake we can reach the head of the snake sitting in Tehran and deal with it as well." Iran is a close ally of the Syrian government.

His remarks came a day after the United States accused Syria of mass killings at a prison near Damascus. Syria denied the allegations.

Galant says a "genocide" is underway in Syria, where an estimated 400,000 people have been killed and half the population has been displaced by a six-year-old civil war.

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3:30 p.m.

Syrian activists say government forces are escalating attacks on opposition-held areas protected under a recently brokered cease-fire agreement.

The Britain-based Syrian Observatory for Human Rights says it has recorded the first fatalities inside the country's four "de-escalation zones" since the agreement came into effect 10 days ago. The deal was brokered by Russia, Turkey and Iran.

The Observatory says two women were killed by rocket fire in a Damascus suburbs and another was died from aerial bombardment in central Homs province.

Local activists reported higher death tolls. Wael Abou Rayan, a media activist in the Homs countryside, says the bombardment of Homs eased since the agreement came into force, but never completely halted.

The reports come as a new round of Syria talks brokered by the United Nations go underway in Geneva.

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2:30 p.m.

Syrian rebels and government officials are meeting separately with the U.N. envoy as peace talks on the war-torn Mideast country got underway in Geneva.

The meetings on Tuesday are the sixth round of the talks brokered by U.N. Special Envoy Staffan de Mistura.

However, the gathering in the Swiss city has been overshadowed by U.S. State Department accusations that the Syrian government built a crematorium in one of its most notorious prisons to cover up dozens of daily executions there.

Monitoring groups, which have noted the mass executions, have not verified the crematorium allegation.

In Geneva, Syria's U.N. Ambassador Bashar Jaafari held a very brief meeting with de Mistura on Tuesday and left without speaking to the media. Opposition representatives are expected to meet with de Mistura later in the day.

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2:15 p.m.

Syrian rebels and government officials have begun their meetings with a U.N. mediator in Geneva to open the sixth round of talks in the last year to wind down the war in Syria. The separate meetings with U.N. Special Envoy Staffan De Mistura are taking place in the shadow of accusations made by the U.S. State Department one day earlier that the that the Syrian government has built a crematorium in one of its most notorious prisons to cover up a routine of dozens of executions daily.

2:15 p.m.

Syria's government says it "categorically" denies U.S. accusations of mass killings at a prison, including executions of political opponents, and burning the victims in a crematorium to hide the evidence.

The Foreign Ministry in Damascus describes the U.S. State Department accusations as "a new Hollywood plot" and "lies" that are being used to justify U.S. "aggression and intervention."

The statement was released on Tuesday.

Western monitors and watchdog groups say they have accumulated evidence of mass killings in Syrian government prisons. However, there hasn't been any substantiated allegation so far of the use of a crematorium

On Monday, the State Department said it believes about 50 detainees are being hanged each day at the Saydnaya military prison, which is located about a 45 minutes' drive north of the capital, Damascus.