Russia is committed to arranging a peace conference on the Syria conflict but other countries and groups are complicating matters by trying to set preconditions, Foreign Minister Sergei Lavrov said on Friday.

Lavrov, who will meet U.S. Secretary of State John Kerry next week to discuss the planned conference, also said shipments of weapons to rebels fighting Syrian President Bashar Assad "contradict the concept of the conference".

Russia, which has backed Assad by sending Damascus arms and protecting him from UN Security Council resolutions, agreed with Washington in May to help try to bring the warring sides to a peace conference. But preparatory talks this week in Geneva between Russian, U.S. and UN officials made no headway.

"The opposition, which is supported by the West, and other countries in the region announced they are not going to the conference as long as the regime doesn't agree to capitulate," Lavrov said after talks with Morocco's foreign minister.

He underlined that when the joint Russian-American initiative was rolled out, it was agreed that the participants would not be allowed to set any preconditions.

No date has been set for the conference. Russia, which opposes external intervention in the crisis, says it is not defending Assad but says his removal from power cannot be a condition for the talks to take place.