A U.N. human rights panel expressed alarm Friday at reports it has received of Syrian security forces torturing children.
The Committee Against Torture said it had received "numerous, consistent and substantiated reports" of widespread abuse in the country.
The chair of the panel, Claudio Grossman, told reporters in Geneva that the reports referring to the abuse of children were of "particular concern."
The U.N. human rights office says more than 3,500 people have been killed in the eight-month uprising against Syrian President Bashar Assad's regime.
Meanwhile, Syria ignored a deadline imposed by the Arab League to allow an observer mission into the country or face economic sanctions, a senior Arab League diplomat said Friday.
The diplomat said the Friday afternoon deadline passed with no word from Damascus. He spoke on condition of anonymity because he was not authorized to speak to the media.
The 22-nation bloc had given Syria 24 hours to agree to the observer mission, saying it would meet to decide on punishing measures that could include a freeze on financial dealings and assets if the deadline was missed.
Patience 'running out'
Syria is the scene of the deadliest crackdown against the Arab Spring's eruption of protests and international pressure has been mounting on Assad to stop the bloodshed.
Earlier Friday, before the deadline passed, Turkey's foreign minister Ahmet Davutoglu said Syria faced a test of goodwill over the proposal and said the country "must open its doors to observers."
Davutoglu said the patience of Turkey and Arab countries was "running out over the bloodshed in Syria."
He spoke during a joint news conference with Italy's new Foreign Minister Giulio Terzi in Istanbul.
Terzi described the situation in Syria as a "worrying tragedy."
Syria had previously slammed the Arab League's ultimatum, which increased the international pressure on Assad's government following France's proposal for "humanitarian corridors" to be set up to alleviate civilian suffering.
However, Russia, China and their partners in the BRICS group of emerging economies warned against foreign intervention without U.N. backing and urged Assad to start talks with the opposition.Story: 5 children among 23 civilians killed in Syria, rights group says
Under an Arab League initiative, Syria had agreed to withdraw troops from urban centers, release political prisoners, start a dialogue with the opposition and allow monitors and international media into the country.
Since then hundreds of people, including civilians, security forces and army deserters, have been killed as the unrest which the U.N. says has claimed at least 3,500 lives since March continued unabated.
The Syrian Observatory for Human Rights, a British-based opposition group, said at least 47 people were killed in Syria Thursday, including 16 soldiers and 17 army deserters, mostly around the city of Homs and Rastan to the north.
"In the case that Syria does not sign the protocol ... or that it later violates the commitments that it entails, and does not stop the killing or does not release the detainees ... (Arab League officials) will meet on Saturday to consider sanctions on Syria," the Arab ministers said in a statement.Story: Army defectors threaten to transform Syrian uprising into civil war
Possible sanctions, which are not intended to affect ordinary Syrians, included suspending flights to Syria, stopping dealings with the central bank, freezing Syrian government bank accounts and halting financial dealings.
They could also decide to stop commercial trade with the government "with the exception of strategic commodities so as not to impact the Syrian people," the statement said.
Syria's economy is already reeling from the eight months of unrest, aggravated by U.S. and European sanctions on oil exports and several state businesses.
The Arab League suspended Syria's membership two weeks ago, while this week the prime minister of neighboring Turkey — a NATO member with the military wherewithal to mount a cross-border operation — told Assad to quit and said he should be mindful of the fate of fallen dictators such as Adolf Hitler, Benito Mussolini and Libya's deposed leader Moammar Gadhafi.
"The Syrian crisis may or may not have entered its final phase, but it undoubtedly has entered its most dangerous one to date," the International Crisis Group said.
Reuters and The Associated Press contributed to his report.