President Barack Obama said the fight against violent extremism is a "battle for hearts and minds," as he called for unity on the second day of a conference bringing together 60 countries Wednesday.

Stressing that the fight against groups like Islamic State cannot be won with military might alone, Obama is expected to talk about "discrediting the voices of extremism" within communities and across the world.

A U.S. diplomat underestimated Lebanon's decision to boycott a conference expected to be held in Washington after Beirut claimed that it cannot take part in a meeting to confront terrorism with the Jewish state's participation.

“Cooperation between the Lebanese and U.S. security agencies is ongoing,” the diplomatic source said in comments published in al-Liwaa newspaper on Wednesday.

Tens of thousands of Danes have gathered for a torchlit vigil in Copenhagen to commemorate the victims of two shootings that shocked the nation and heightened fears of a new surge in anti-Semitic violence.

Stunned citizens in what is usually one of the world's most peaceful countries flocked to Monday's rally in a square near the cultural center where the first attack took place. Many held flaming torches aloft, illuminating the chilly winter night.

Iran has denied reports that supreme leader Ayatollah Ali Khamenei wrote to U.S. President Barack Obama in response to an October letter mooting cooperation against the Islamic State group.

The Wall Street Journal reported on Friday that Khamenei, who has the final say in all matters of state in Iran, had sent a secret but noncommittal letter to Obama in response to his overture.

Iran's supreme leader has sent a secret but noncommittal letter to U.S. President Barack Obama in response to American overtures, amid talks to strike a nuclear accord, The Wall Street Journal reported Friday.

The newspaper cited an Iranian diplomat as saying Ayatollah Ali Khamenei had written to Obama in response to the president's letter sent in October.

U.S. President Barack Obama on Friday condemned the "brutal and outrageous" execution-style murders of three Muslim students in North Carolina at the hands of a neighbor who espoused anti-religious views.

"No one in the United States of America should ever be targeted because of who they are, what they look like, or how they worship," Obama said in a statement.

A U.S.-led coalition battling Islamic State extremists in Iraq and Syria is "on the offensive" and will prevail, President Barack Obama vowed Wednesday after asking Congress to back the war effort.

Obama also said that the United States "should not get dragged back into another prolonged ground war in the Middle East," but noted that he would be ready to deploy special forces in specific situations.

President Barack Obama on Wednesday asked Congress to back a global war against the Islamic State group, albeit with curbs on his ability to send in U.S. ground forces.

Tantamount to a declaration of war, the authority would provide Obama political cover at home and a firmer legal basis on which to prosecute the fight.

Western governments evacuated their diplomats from Yemen on Wednesday as conflict deepened in the impoverished Saudi neighbour which has long been on the front line of the war against al-Qaida.

Exactly four years after the start of Arab Spring-inspired protests that forced veteran strongman Ali Abdullah Saleh from power, his Western-backed successor is under effective house arrest, after being ousted by Shiite militia last week.

U.S. President Barack Obama called his Russian counterpart Vladimir Putin to discuss escalating violence in eastern Ukraine and "Russia's ongoing support for the separatists there," the White House said Tuesday.

During the call Obama reiterated America's support for Ukraine's "sovereignty and territorial integrity," National Security Council officials said.