US Secretary of State John Kerry was to leave for Saudi Arabia on Friday to launch a week of efforts to try to end the crises in Libya and Syria.

From Jeddah, where he will meet senior Saudi leaders, Kerry will fly on Monday to Vienna where he will co-host international meetings on the two conflicts.

Then on Wednesday, he will fly on to Brussels for the Nato foreign ministers' meeting and talks on the full range of challenges facing the Western allies.

Kerry's spokesman John Kirby said the secretary of state and Italian Foreign Minister Paolo Gentiloni would jointly host the Libya crisis meeting.

Attendees will "discuss international support for the new Government of National Accord, with a focus on security," Kirby said.

Libya's new UN-backed government has been set up to unite the fractured country and fight the Islamic State group, but it is still a work in progress.

Officials say the fledgling regime is drawing up a list of requests for Western partners to assist its forces with arms, training and intelligence.

After the Libya meeting, Kerry and Russian Foreign Minister Sergei Lavrov will lead a meeting of the 17-nation International Syria Support Group.

Kirby said the goal was to "ensure humanitarian access throughout the country, and to expedite a negotiated political transition in Syria."

The ISSG, under the odd couple of Kerry and Lavrov, is pushing Syrian President Bashar al-Assad's regime and a coalition of opposition groups to respect a shaky truce.

Officials hope next week's meeting will inject new life into the peace process and - if the ceasefire holds - secure talks on forming a unity government.

And, with Russia and France, Kerry will also co-host a meeting on the Nagorno-Karabakh conflict with the rival Armenian and Azerbaijani presidents.