British police received a bomb threat for London from dissident Irish republicans Monday, on the eve of a historic trip by Queen Elizabeth II to Ireland.

The warning heightened tensions after a spike in violence in Northern Ireland and a statement by a top paramilitary group that the British monarch was wanted for "war crimes."

President Barack Obama Friday announced his "tireless" envoy to the Middle East, George Mitchell, was resigning, but vowed the U.S. administration remained committed to the faltering peace process.

Describing Mitchell as having taken on "the toughest job imaginable," Obama paid tribute to the veteran diplomat who had "worked grueling hours to advance the interests of the United States and the cause of peace."

Kenya has tightened security around the home of U.S. President Barack Obama's step-grandmother after the killing of Osama bin Laden, an official said Thursday.

"In the wake of security challenges including terror threats, I can confirm that we decided to enhance security at the home of Mama Sarah ...," said regional administrator Francis Mutie, using the popular name for Obama's step-grandmother who lives in western Kenya.

U.S. President Barack Obama's long-expected speech on turmoil in the Arab world and North Africa could take place as soon as next week, in the aftermath of the death of Osama bin Laden.

Obama's address will focus on the popular uprisings, violence and political change sweeping the region, and also come at a time of deep uncertainty for his hopes of renewing talks between Israel and the Palestinians.

The leader of al-Qaida's Yemen branch has warned that jihad will become more "intense and harmful" after the killing of Osama bin Laden by U.S. commandos, SITE monitoring group reported Wednesday.

Nasir al-Wahishi, leader al-Qaida in the Arabian Peninsula (AQAP), said in a statement posted on an Islamist website that the "ember of jihad is brighter" following the May 2 death of bin Laden, the U.S.-based group said.

The United States warned Friday it would take "additional steps" against Syria if it continues a brutal crackdown on protesters, a week after imposing tough sanctions on the Arab nation.

"The United States believes that Syria's deplorable actions toward its people warrant a strong international response," the White House said in one of its strongest statements yet since the outbreak of unrest there.

Cuban leader Fidel Castro called the death of Osama bin Laden an "abhorrent" assassination, warning in an article Thursday that it would reap "hatred and vengeance" against the United States.

"Whatever actions may have been attributed to bin Laden, the assassination of an unarmed human being surrounded by his family constitutes an abhorrent act," he said, writing in the Cuban press.

U.S. President Barack Obama paid solemn tribute at the site of the downed Twin Towers, days after 9/11 architect Osama bin Laden was shot dead by American commandos in his Pakistani lair.

Intel found in bin Laden's compound revealed al-Qaida was considering a possible train attack at an unspecified location in the United States as a grim marker of this year's 10th anniversary of the September 11 attacks.

A U.S. drone attacked a Saudi al-Qaida leader in southern Yemen on Thursday, but missed and killed two local al-Qaida members, a security source and witnesses told Agence France Presse.

The drone had targeted the Saudi as he drove to the home of local al-Qaida men, the security source said, asking not to be identified.

President Barack Obama decided Wednesday not to release photos of Osama bin Laden's corpse, citing national security risks and saying the United States should not brandish "trophies" of its victory.

Obama's war cabinet had been debating whether to publish gruesome post-mortem photos of the al-Qaida terror chief, who was gunned down by U.S. Special Forces in a covert raid inside Pakistan on Sunday.