(Reuters) - President Barack Obama told G8 leaders meeting at Camp David that Syrian President Bashar al-Assad must leave power, and pointed to Yemen as a model of how political transition could work there, the White House said on Saturday.

The Group of Eight leaders, in a statement summing up their discussions, urged all parties in Syria to adhere to their commitments under a joint U.N.-Arab League peace plan "including immediately ceasing all violence so as to enable a Syrian-led, inclusive political transition leading to a democratic, plural political system."

Ben Rhodes, an Obama deputy national security adviser, said the recent focus on securing access for U.N. monitors and keeping track of ceasefires had distracted from the fundamental problems in Syria, where Assad, whose father ruled the country before him, has been attacking protesters for 14 months.

The United Nations estimates some 9,000 people have been killed in Syria since the start of the uprising in March 2011, when unrest that toppled leaders in Egypt, Tunisia and elsewhere was spreading across North Africa.

Washington´s patience with Assad has been wearing thin. Assad said he would adhere to a U.N.-Arab League peace plan but has failed to bring violence to a full halt, blaming "terrorists" for recent attacks in Damascus and elsewhere.

"It is our assessment that you are not going to be able to solve this problem just with monitors and ceasefires, that you need to have a political process under way that is responsive to the Syrian people, because otherwise you are not going to solve the problem," Rhodes said.

He said the G8 leaders - from Britain, France, Germany, Italy, Japan, Russia and Canada, plus the United States, which is hosting the summit - discussed during their dinner on Friday how a political transition could take place in Syria.

" We remain appalled by the loss of life, humanitarian crisis, and serious and widespread human rights abuses in Syria," the G8 statement said. "Use of force endangering the lives of civilians must cease."

Alone among the eight, Russia has supported Assad and opposed stiffer U.N. sanctions.

"Some may like or dislike the Syrian government, some may have different views on the last election which took place in Syria but one cannot avoid a question - if Assad goes, who will replace him?" said Mikhail Margelov, a Russian parliamentarian and aide to Prime Minister Dmitry Medvedev.

"We believe that the Syrian crisis cannot be dealt with an ax. One should work on it with a pair of pincers," he said.

The G8 statement said the leaders welcomed the deployment of the U.N. mission "and urge all parties, in particular the Syrian government, to fully cooperate with the mission. We strongly condemn recent terrorist attacks in Syria."

Obama brought up Yemen as an example of a leader departing power peacefully and ushering in a democratic process, Rhodes said. "Our point was that we need to see political transition under way that brings real change to Syria," he said.

"We believe that change has to include Bashar al-Assad leaving power. And unless you begin the process of a political transition of some sort, you are not going to be able to deal with reducing the violence and addressing the grievances of the people who came out in the street to start with," Rhodes said.

Former Yemeni President Ali Abdullah Saleh ruled the poor Gulf nation for 33 years and was unseated after an uprising last year that split the country´s armed forces into warring factions.

Saleh was granted immunity from prosecution over the killing of protesters as part of power transfer deal that eased him out of office. Many Yemenis believe Saleh ought to have been put on trial; rights groups say hundreds of protesters were killed by his security forces in the revolt.

U.N. Security General Ban Ki-moon said this month there was only a narrow window of opportunity to avert full-scale civil war in Syria, which borders Turkey, Jordan, Israel, Iraq and Lebanon. The country´s 23 million people comprise a mix of sects and ethnic groups whose tensions could resonate in the region.

(Additional reporting by Gleb Bryanski. Writing by Laura MacInnis; editing by Warren Strobel, Christopher Wilson and Vicki Allen) (© Thomson Reuters 2012. 05/19/12)