Syrian President Bashar al-Assad has said he has no intention of giving up power and the issue is not up for discussion.

"If we wanted to give up, we would have done so at the very beginning. We are on guard for our country. This issue is not up for discussion," he was reported as telling visiting Russian politicians.

Mr Assad’s comments came as US Secretary of State John Kerry praised a decision by Syria's opposition leaders to attend an internationally sponsored peace conference that aims to bring an end to the war there.

"This is a courageous vote in the interests of all the Syrian people who have suffered so horribly under the brutality of the Assad regime and a civil war without end," Mr Kerry said in a statement.

He called the opposition decision to attend next week's Geneva II gathering "a path that will ultimately lead to a better future for all Syrians".

The Geneva II conference aims to find a way out of almost three years of brutal conflict that has claimed the lives of 130,000 people and forced millions from their homes.

More than 35 countries will gather in the Swiss cities of Montreux and Geneva from Wednesday for talks on setting up a transitional government to lead the country, in line with a 2012 deal.

Syria's opposition chief Ahmad Jarba said yesterday that his opposition group has agreed to attend an international peace conference only to secure the removal of President Bashar al-Assad from power.

Mr Kerry promised Washington's continued backing of the opposition in the high stakes talks.

"The United States will continue to support the Syrian opposition as they embark on the best opportunity to achieve a negotiated political transition as outlined in the Geneva communique, including the establishment, based on mutual consent, of a transitional governing body exercising full executive authority, including over military and security entities," he said.

The top US diplomat added that Washington will push to bring to an end the worst abuses of the Assad regime, including its use of "SCUD missiles, barrel bombs, and horrific weapons used against civilians".

The United States, he said, also will press next week for "improved humanitarian access, prisoner releases, and the return of journalists and aid workers held hostage".

Mr Kerry added: "We all know that the process ahead will be difficult, but I say directly to the Syrian people: we will stand by you every mile of the journey as you seek to achieve the freedom and dignity that all Syrians deserve."