A young couple behind a mass shooting in California that left 14 dead had amassed a huge arsenal and carefully planned their attack, authorities said Thursday, refusing to rule out terrorism.

The suspects -- Syed Farook, 28, and his 27-year-old wife Tashfeen Malik -- were killed in a shootout with police after Wednesday's rampage at a holiday party in San Bernardino, about an hour's drive east of Los Angeles.

Over 1,500 migrants have been rescued off the Libyan coast in seven separate operations, the Italian navy said Thursday, after a break in bad weather sparked fresh attempts at the perilous Mediterranean crossing.

The migrants were plucked from their boats by the Italian coastguard, the navy, a Doctors Without Borders boat and two vessels operating under the EU's Operation Sophia, which patrols the sea for people smugglers.

Chechen leader Ramzan Kadyrov on Thursday confirmed the man executed by the Islamic State group in a recent video was a Chechen from Russia but dismissed claims he was a Russian spy.

"This Russian is a Chechen. He was decapitated. These are confirmed facts," Kadyrov said in comments on his official website. 

The foreign ministers of Turkey and Russia met in Belgrade on Thursday in the first high-level bilateral talks since the downing of a Russian warplane by Turkish jets sparked an unprecedented crisis. 

The meeting between Russia's Sergei Lavrov and Turkey's Mevlut Cavusoglu was held on the sidelines of the ministerial council of the Organization for Security and Cooperation in Europe.

Azerbaijan has killed five "radical Islamists" and arrested 32 others for allegedly plotting a coup in the oil-rich ex-Soviet country, Interior Minister Ramil Usubov has said.

"During a special operation in the village of Nardaran, 32 radical Islamists were arrested and five were killed," Usubov said in a televised statement on Wednesday night.

A proposed change to France's constitution would not place a time limit on states of emergency of the kind introduced after the Paris attacks, according to a draft text seen by AFP on Thursday.

However, the draft change to the constitution says exceptional measures taken during a state of emergency -- such as powers of house arrest -- could be prolonged "for a maximum period of six months" after the original state of emergency has expired.

President Recep Tayyip Erdogan on Thursday said Turkey had proof Russia was involved in illegal oil trade with the Islamic State (IS) group in Syria, countering "immoral" Russian allegations that his own family was importing oil from the jihadists.

"We have the proof in our hands. We will reveal it to the world," Erdogan said in a televised address in Ankara.

A trial of 35 members of Basque separatist parties including Batasuna, the banned political wing of the armed ETA group, got underway Thursday in Spain after being repeatedly postponed.

The defendants stand accused of carrying out political activities to back ETA, blamed for the deaths of 829 people in a four-decade campaign for an independent Basque homeland in northern Spain and southwest France.

Brazilian President Dilma Rousseff's party was appealing Thursday to the Supreme Court to block impeachment proceedings, in the first counter-blow of a battle plunging Latin America's biggest country into political turmoil.

The morning after the speaker of the lower house of Congress triggered the impeachment process against Rousseff on grounds that she illegally manipulated government accounts, her Workers' Party sprang into action.

Hungary has launched a vitriolic media campaign attacking the EU's plan to distribute 160,000 asylum-seekers among member states, and warned it would file a legal challenge against the quota scheme later Thursday.

Full-page messages appeared in national newspapers with the headline: "The quota increases the terror threat!" against a black background.