New York based Human Rights Watch said Ghiyath Matar, 26, had been a key player in organising protests against President Bashar al-Assad's regime.

Arrested on September 6, he died in detention after being tortured the group added citing activists, who said he had bruises on his chest and signs of facial injuries.

"The United States condemns in the strongest possible terms the killing of Syrian human rights activist Ghiyath Matar while in the custody of Syrian security forces," US State Department spokeswoman Victoria Nuland said.

"His brave commitment to confronting the regime's despicable violence with peaceful protest serves as an example for the Syrian people and for all those who suffer under the yoke of oppression," she added in a statement.

The Syrian Observatory for Human Rights reported that a youth shot at Matar's funeral in the capital Damascus had died of his injuries Sunday.

The Observatory told AFP in Nicosia that the 17-year-old youth had died after having been wounded on Saturday when security forces opened fire on mourners at the funeral in Daraya, near Damascus.

It also said that a woman had been shot dead in eastern Syria on Sunday.

"A 40-year-old woman was killed at noon on Sunday by a stray bullet as security forces were tracking wanted people in the town of Bukamal," the Britain-based rights group cited an activist in Deir Ezzor province as saying.

Also on Sunday, the Observatory said prominent rights campaigner Najati Tayara was in a "very bad" condition after having been severely beaten at a Homs prison in central Syria.

It said in a statement received by AFP in Nicosia that it "has learned that investigators at the so-called 'Polish' prison in Homs severely beat Tayara on Friday.

Tayara, 66, was first arrested in Homs on May 12, a day after he reported that shelling and gunfire had rocked the city, the country's third largest.

According to the Observatory, security forces have arrested "more than 70,000 people" in their crackdown on anti-regime protests that erupted in mid-March, and that "15,000 of them are still in detention."

The latest deaths came after security forces killed at least 12 people on Saturday in their ongoing bid to crush anti-regime dissent, activists said.

As the crackdown continued, the Arab League on Saturday announced it had reached an agreement with Assad on long-promised reforms.

Arab League chief Nabil al-Arabi met Assad on Saturday with a 13-point document outlining Arab proposals to broker an end to the bloodshed, hold elections and push for reforms.

During his talks with Assad, Arabi said the League and other Arab countries "reject any form of foreign interference in Syrian domestic affairs," the official news agency SANA reported.

Back in Cairo, Arabi said the agreed reforms would be submitted to the council of the Arab League on Monday.

On Friday, thousands of protesters for the first time called for international protection against repression.

But SANA reported Assad as saying that there was a need "to not get caught in campaigns of disinformation against Syria."

He denounced the campaign of spreading "wrong facts" aimed in his opinion "to harm the image of Syria and destabilise" the country, the agency reported.

On Friday, US State Department spokeswoman Nuland said Washington planned to speed up work on a UN Security Council resolution targeting Syria.

"We're looking at accelerating that work next week," she said, referring to a draft resolution likely to include sanctions.

The United States and several European powers are pushing for a UN resolution to condemn Assad's regime which has already been slapped with sanctions, including an oil ban.

But they have met with stiff resistance from Russia, China and a group of emerging nations including Brazil.

The United Nations says more than 2,200 people -- mostly civilians -- have been killed in a crackdown on almost daily protests by pro-democracy and anti-regime demonstrators in Syria since mid-March.

But Damascus insists that it is battling "armed terrorist gangs."

Meanwhile, the six-nation Gulf Cooperation Council issued a statement urging Syria to immediately stop its "killing machine" against anti-regime protesters.

Meeting in Jeddah in Saudi Arabia, the group of oil-rich Arab monarchies also urged "the immediate implementation of serious reforms that meet the aspirations of the Syrian" people.