An unprecedented effort to destroy Syria's most dangerous chemical agents aboard a U.S. naval ship in the Mediterranean is making progress, the Pentagon said Friday.

The American specialists on the MV Cape Ray so far have neutralized about a quarter of the chemical DF, a precursor used to make lethal Sarin gas, officials said.

Jihadists from the Islamic State group killed at least 50 Syrian soldiers in an ambush on Friday, summarily executing most of them after their capture, a monitoring group said.

The assault in Raqa province, the jihadists' Euphrates valley stronghold, came amid their fiercest assault yet on Damascus-held territory.

Jewish museums in Norway remained closed to the public on Friday, the day after Norway stepped up security as it announced a possible imminent terrorist attack by jihadists coming from Syria.

The Jewish community seems like a probable target for Islamist militants who have fought in the war-torn country, as underscored by the deadly attack on Brussels' Jewish Museum in May.

Campaigners on the Greek island of Crete launched a seaborne protest on Friday against the destruction of Syrian chemical agents in the Mediterranean, the state-run ANA agency reported.

Some 36 people including local mayors, labor unionists and a leftist MP sailed from the port of Hania aboard three sailboats.

A new power struggle has emerged within the Syrian rebellion after Al-Qaida announced it aims to create an Islamic "emirate" to compete with rival jihadist group, the Islamic State (IS).

As with IS in its early days, when it was still known as the Islamic State of Iraq and the Levant, Al-Qaida affiliate the Al-Nusra Front is spreading its zone of influence, taking over strategically located villages, and competing with other rebel groups.

An offensive by the jihadist Islamic State group against Syrian government-held territory has killed at least 74 people in the past 24 hours, a monitoring group said on Friday.

Buoyed by their capture of swathes of western and north-central Iraq last month launched multiple attacks on army positions across northern and northeastern Syria on Thursday.

Jordan shot down a drone on Friday as it flew over the northeastern region of Mafraq, near the border with Syria, a security official said.

In the first such incident since the conflict erupted in neighboring Syria in 2011, "the Jordanian armed forces shot down an unidentified drone with a missile as it flew over Mafraq," the official told AFP.

Syrian government forces have stepped up attacks on hospitals and clinics, U.N. chief Ban Ki-moon said Thursday, accusing all sides of blocking aid deliveries as a "tactic of war."

Ban told the U.N. Security Council in a report that both government forces and rebel fighters were increasingly targeting vital services such as water stations and electric cables, compounding the humanitarian crisis.

The jihadist Islamic State group launched multiple attacks on government-held territory across northern and northeastern Syria on Thursday, a monitoring group said.

The Syrian Observatory for Human Rights said it was the first time that IS, accused by rival rebel groups of focusing its formidable firepower more on them than on the Damascus regime, had launched such coordinated assaults on army positions.

Around 700 tons of Syria's chemical agents have been delivered to specialized destruction facilities in Britain, Finland and the U.S., the watchdog in charge of the mission said Thursday.

"All 1,300 metric tonnes of chemicals removed from Syria by the international maritime operation have been delivered to destruction facilities outside the country," the Organization for the Prohibition of Chemical Weapons (OPCW) said in a statement.