The Syrian regime on Tuesday accused U.N. human rights chief Navi Pillay of "flagrant bias" a day after she warned violations in the war-torn country had reached "horrific dimensions".

The Syrian delegation to the U.N.'S Human Rights Council "strongly denounced the flagrant bias of Navi Pillay over the situation" in Syria, state news agency SANA reported.

Twenty people implicated in a deadly attack on the U.S. embassy in the Tunisian capital last year, some of whom could face the death penalty, insisted they were innocent as their trial opened on Tuesday.

Hundreds of angry Islamist protesters attacked the U.S. mission in Tunis on September 14 after an American-made film mocking their religion was published on the Internet.

Soldiers treated "wounded" civilians on Tuesday as Israel simulated a chemical weapons attack as part of its annual civil defense drill, which comes against a backdrop of tension on its borders with Syria and an increasingly unstable Lebanon.

In the city of Holon, south of Tel Aviv, 150 troops evacuated civilians to a field hospital set up in a sports stadium and "contained" the area, simulating a chemical warhead attack, an army spokeswoman told Agence France Presse.

A senior French diplomat paid a brief visit to Tehran to discuss "solutions" to end the bloody crisis in Syria, Iranian foreign ministry spokesman Abbas Araqchi said.

The director of the North Africa and Middle East section of the French foreign ministry, Jean-Francois Girault, met on Monday with Iranian deputy foreign minister Hossein Amir Abdollahian, Araqchi said.

Attacks killed 16 people in Iraq on Tuesday, officials said, as the cabinet discussed how to curb violence that has left over 500 dead this month and raised fears of all-out sectarian conflict.

The U.N. envoy to Iraq meanwhile urged the country's feuding leaders to meet to resolve long-running political crises that have paralyzed the government and been blamed for its inability to reduce the violence.

Violence in Iraq has killed more than 500 people in May, Agence France Presse figures showed on Tuesday, as authorities struggled to contain a wave of unrest that has raised fears of all-out sectarian conflict.

And the U.N. envoy to Iraq urged the country's leaders to meet to resolve long-running political crises that have paralyzed the government and been linked to its inability to reduce the violence.

Hundreds of Yemenis staged a sit-in on Tuesday demanding the release of dozens of activists detained during the uprising against former president Ali Abdullah Saleh, some of whom have gone on hunger strike.

Activists and relatives gathered for the second consecutive day outside the office of the public prosecutor, whom they hold responsible for keeping the detainees behind bars since 2011, an Agence France Presse correspondent reported.

Israel "will know what to do" if Russia delivers anti-aircraft missiles to Syria, its defense minister said Tuesday, in an apparent allusion to another air strike on the war-torn neighboring country.

"The deliveries have not taken place, and I hope they do not. But if, by misfortune, they arrive in Syria, we will know what to do," Moshe Yaalon said.

At least 15 prisoners were killed over the weekend in clashes between rebels and troops at the central prison in Syria's northern town of Aleppo, a watchdog said on Tuesday.

Citing phone calls from sources inside the prison, the Syrian Observatory for Human Rights said "at least 15 prisoners were killed in shelling of the prison, which rebel fighters have besieged."

Syria's opposition on Tuesday denounced as "too little, too late" an EU decision to lift an arms embargo on rebels fighting the regime of President Bashar Assad.

"Definitely it is a positive step, but we are afraid it could be too little, too late," Louay Safi, spokesman for Syria's main opposition National Coalition, told Agence France Presse.