The White House renewed calls Saturday for Bashar Assad to step down at a "critical juncture" in Syria after U.N. observers suspended their mission, saying it was discussing the way ahead with allies.

The unarmed observers have been targeted almost daily since deploying in mid-April to monitor a U.N.-backed but widely flouted ceasefire brokered by international envoy Kofi Annan, and they were likened to "sitting ducks in a shooting gallery" by Susan Rice, the U.S. envoy to the United Nations.

Russia cautioned Western powers on Thursday against launching "hasty" wars that could lead to the rise of radical Islamist factions and even result in regional nuclear wars.

Prime Minister Dmitry Medvedev told a legal forum ahead of his visit to the G8 summit as Russia's official representative at Camp David that Moscow noted numerous examples of powers infringing on the sovereignty of other states.

Presumptive Republican nominee Mitt Romney won primary ballots in Oregon and Nebraska on Tuesday, partial results showed, moving him a step closer to winning his party's formal White House nomination.

Romney won 73 percent of the vote in Oregon against 12.2 percent for Ron Paul, who suspended active campaigning this week, and 5.7 percent for Newt Gingrich, according to results from nearly two thirds of precincts.

Russia's President Vladimir Putin will skip the G8 summit and ice breaker talks with President Barack Obama next week, the White House said Wednesday, raising new questions about Moscow-Washington ties.

Putin had been expected at the summit at Camp David and to hold talks with the U.S. leader in a first test of whether the "reset" policy between the Kremlin and the White House will survive his return to the presidency.

Newt Gingrich has decided to quit the U.S. presidential race, U.S. media reported Wednesday, paving the way for former Massachusetts governor Mitt Romney to be crowned the Republican nominee.

Gingrich, former speaker of the House of Representatives, is expected to hold a final campaign event in Washington on Tuesday May 1, formally ending his White House bid and throwing his support behind his former opponent.

Syrian forces on Tuesday pounded protest hubs in apparent breach of a ceasefire deal brokered by U.N.-Arab League envoy Kofi Annan, who appealed for his plan to be implemented without preconditions.

Syria said it was abiding by the plan, but Annan accused Damascus of pulling troops from some areas and moving them to others, and the rebel Free Syrian Army warned it would resume attacks if the government offensive does not stop.

Thousands of protesters gathered outside the White House Saturday to demand that the United States "stop the massacre in Syria," where an estimated 8,000 people have been killed in a regime crackdown.

Wearing T-shirts declaring "I have a dream of a free Syria" and "No longer afraid," the demonstrators -- who numbered 4,000, according to organizers -- were marking the first anniversary of a bloody revolt against President Bashar Assad's regime.

The White House said Monday that U.S. strategy would not change in Afghanistan despite the fallout from a U.S. soldier's shooting rampage which killed 16 civilians in a new jolt to America's war policy.

"Our strategic objectives have not changed and they will not change," White House spokesman Jay Carney said, adding Washington remained committed to disrupting, disarming and defeating al-Qaida and training Afghans to ensure their own security.

The United States called Friday on all countries to condemn the "horrific" violence in Syria as President Barack Obama declared that leader Bashar al-Assad's days were numbered.

U.S. officials voiced outrage as the Red Cross said it was unable to gain access to the vanquished rebel stronghold of Baba Amr in Homs, where government forces were reported to be conducting reprisals after a month-long bombardment.

The White House said Tuesday that al-Qaida's efforts to take advantage of violence in Syria mean it is no time to send arms to opponents of Syrian President Bashar Assad.

"Without getting into assessments of our intelligence capabilities, I would simply say that we are aware of the fact that al-Qaida and other extremists are seeking to take advantage of the situation created by Assad's brutal assault on the opposition," said White House spokesman Jay Carney.