Two heavily-armed suspects, a man and a woman in their 20s, have died in a shootout with police after killing 14 people at a Christmas party in California in America's deadliest mass shooting in three years.

Police identified the two as Syed Farook, a 28-year-old U.S. citizen who worked for the local county, and Tashfeen Malik, 27, whose nationality was unknown. They said the suspects were either married or in a relationship.

Brazil's lower house speaker triggered impeachment proceedings against President Dilma Rousseff on Wednesday, setting the stage for a political battle that could see the country's first female leader forced from office.

"It's to authorize the initiation (of impeachment), not to judge on its merits," Speaker Eduardo Cunha told journalists after officially accepting an impeachment petition, which now goes to a committee which will decide whether or not to authorize a trial.

Donald Trump has extended his lead in the Republican presidential nomination race amid a fresh bout of controversy over his remarks about Muslims, while Hillary Clinton has consolidated her lead among Democrats, a poll showed Wednesday.

Less than two months before the first state-wide votes to determine the presidential nominees, Trump was his party's undisputed leader, backed by 27 percent of registered Republican voters, while his closest challenger for months, retired neurosurgeon Ben Carson, has faded to third place, according to the Quinnipiac University survey.

Germany is seeking an agreement with Turkey to deploy six reconnaissance aircraft at a base close to Syria as Ankara gave France the go-ahead to use its airspace in the fight against the Islamic State group, diplomatic sources said Wednesday.

As Berlin steps up its role in the anti-IS fight, Turkey and Germany were working to finalize a memorandum of understanding to agree within a legal framework the deployment of six Tornado reconnaissance aircraft as well as one refueling aircraft at the Incirlik air base, a source in Ankara said.

The United States strongly denied Wednesday a Russian claim that the Turkish government buys smuggled Syrian oil from the Islamic State jihadist group.

A State Department spokesman admitted that there was a longstanding issue of oil being illegally transported to Turkey from wells in what is now IS territory.

The United States said Wednesday that it was ready to take the next step in implementing the Iran nuclear deal after an IAEA report on Tehran's weapons program.

State Department spokesman Mark Toner said that the U.N. watchdog had confirmed Washington's long-standing allegation that Tehran had once been working on a nuclear bomb, but he added that Iran had cooperated adequately with IAEA investigators.

Police were hunting Wednesday for one or more gunmen who opened fire inside a building in San Bernardino in California, with reports of 20 victims.

Heavily armed SWAT teams, firefighters and ambulances swarmed the scene, located about an hour east of Los Angeles, as police warned residents away.

President Recep Tayyip Erdogan accused Russia of "slander" on Wednesday over Moscow's allegations Turkey had bought oil from Islamic State (IS) jihadists in Syria.

"No one has a right to engage in slander against Turkey by saying that Turkey is buying oil from Daesh (IS)," Erdogan said in comments broadcast by Turkish television on a visit to Qatar.

Afghan Taliban leader Mullah Akhtar Mansour was seriously injured in a firefight following a heated argument at a meeting of militant commanders, officials said Wednesday, exposing deepening divisions within the fractious militant movement.

It was unclear if he survived the clash, which occurred just four months after Mansour was appointed Taliban supremo in a deeply acrimonious leadership succession.

Spain's Constitutional Court on Wednesday revoked a resolution passed by Catalonia's parliament that declared the start of a secession process in the wealthy northeastern region, Prime Minister Mariano Rajoy said.

"It was a unanimous decision," the conservative prime minister said during a speech.