Rights group Amnesty International said it had uncovered evidence of the gruesome death of an 18-year-old girl in custody in Syria as the EU and Switzerland stepped up sanctions against the regime.
Activists reported at least another nine deaths in Syria on Friday, the traditional day of protests by opponents of President Bashar al-Assad.
And Turkey announced it had intercepted an arms shipment at sea destined for Syria.
The Britain-based Syrian Observatory for Human Rights said security forces had shot dead nine people in the Homs area, in the centre of the country, during protests on Friday.
The Local Coordination Committees (LCC) group reported Friday's death toll as 12: eight in Homs including a five-year-old child, two in Duma, one in Zabadani and one in Damascus.
The Observatory also said nearly 2,000 people had demonstrated in the eastern oil hub of Deir Ezzor, calling for Assad's downfall.
State television said "six security agents were wounded in Deir Ezzor by armed terrorist groups."
More than 10,000 demonstrators also gathered in four locations in the predominantly Kurdish province of Hassakeh in the northeast. Protests were also reported in the southern Daraa region.
The office of the UN High Commissioner for Human Rights in Geneva has said the death toll from the crackdown on dissent since March 15 has risen to more than 2,700.
Amnesty said it had compiled the names of more than 2,200 people reported killed during the unrest.
It also said it had uncovered evidence of the gruesome death of an 18-year-old girl, whose mutilated body was discovered in a Homs mortuary last week, two months after her arrest.
She was thought to be the first woman among more than 100 Syrians known to have died in custody since the protests erupted, the London-based watchdog said.
The family of Zainab al-Hosni had discovered her body "by chance" and "in horrific circumstances," while collecting the body of her activist elder brother Mohammad Deeb, said Amnesty.
He had also apparently been tortured and killed in detention. But his sister's body had been decapitated and her arms and skin removed, Amnesty said.
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"We have documented other cases of protesters whose bodies were returned to their families in a mutilated state during recent months," said Philip Luther, Amnesty's deputy director for the Middle East and North Africa.
"But this is particularly shocking."
The latest deaths come as both the European Union and Switzerland announced broader sanctions against Damascus.
The European Union banned new investments in the oil sector and prohibited the delivery of banknotes to Syria's central bank.
The 27-member EU also added two individuals and six companies to a list of people and entities facing an assets freeze and travel ban, a diplomat said on condition of anonymity.
The measures, the seventh set of EU sanctions imposed to punish Assad's regime for its relentless crackdown, came into force on Saturday.
Earlier this month, the EU adopted a ban on crude oil imports which is expected to hit Damascus hard. The EU buys 95 percent of Syrian oil exports, providing a third of the regime's hard currency earnings.
Switzerland's economy ministry announced Friday it was imposing an embargo on the import, sales and transport of Syrian oil and oil products "due to the relentless repression imposed by the Syrian security forces."
Its sanctions also took effect Saturday.
Switzerland has already imposed travel embargoes and asset freezes on 54 individuals and 12 companies connected to the Syrian regime.
Syrian assets frozen in Switzerland currently stand at 45 million francs (37 million euros, $50 million).
Turkey's Prime Minister Recep Tayyip Erdogan announced the interception of what he said was an arms shipment bound for Syria.
"Turkey has arrested a ship flying the Syrian flag and carrying weapons," the Anatolia news agency quoted Erdogan as telling reporters in New York, where he attended the UN General Assembly.
Erdogan did not say when and where the ship was stopped.
On Tuesday Erdogan said he had broken off dialogue with Syria and warned of sanctions, after talks with US President Barack Obama.
Damascus does not accept that popular opposition to the authorities exists, instead blaming "armed gangs" and "terrorists" for trying to sow chaos.