WASHINGTON, Jan. 16 -- U.S. Secretary of State John Kerry on Thursday urged the Syrian opposition to attend a key peace conference which aims at bringing an end to the three-year conflict in Syria.

Kerry called on the Syrian Opposition Coalition to vote favorably at a meeting on Friday to participate in the much- anticipated peace conference in Geneva on Jan. 22.

"We do so knowing that the Geneva peace conference is not the end, but rather the beginning," Kerry said.

The top U.S. envoy said the conference, known as Geneva II, is a process that is the best opportunity for the opposition to achieve the goal of a political solution "for this terrible conflict that has taken many, many, too many lives."

Kerry reiterated that the goal of the conference is to establish a process to form a transitional governing body in Syria to bring about an end to the civil war.

The war has triggered "one of the planet's most severe humanitarian disasters" and created the seeding grounds for extremism, he noted.

He added that any names put forward for leadership of Syria's transition must be agreed to by both the government and the opposition, which is "the very definition of mutual consent."

Kerry also expressed deep concern about the rise of extremism in Syria, which he said is the "strongest magnet for terror of any place today."

Western governments have backed the opposition fighting against the Syrian government but are increasingly concerned about the growing influence of Jihadist groups in Syria.

Media reports said U.S., British and German intelligence agencies have visited Syria to discuss security cooperation with the Syrian government amid worries about Islamist extremists among the opposition.

In a telephone talk on Thursday, U.S. President Barack Obama and British Prime Minister David Cameron discussed the growing security challenge posed by extremist groups in Syria and the region, White House said in a statement.

The leaders also reaffirmed their commitment to the Geneva II process designed to address the growing humanitarian crisis caused by the conflict, said the statement.