GENEVA (AP) — The United States and Russia said Friday they had resolved a number of issues standing in the way of restoring a nationwide truce to Syria and opening up aid deliveries, but were unable once again to forge a comprehensive agreement on stepping up cooperation to end the brutal war that has killed hundreds of thousands.

After meeting off-and-on for nearly 10 hours in Geneva on Friday, U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov could point to only incremental progress in filling in details of a broad understanding to boost joint efforts that was reached last month in Moscow.

Their failure to reach an overall deal highlighted the increasingly complex situation on the ground in Syria – including new Russianbacked Syrian government attacks on opposition forces, the intermingling of some of those opposition forces with an al-Qaida affiliate not covered by the truce and the surrender of a rebel-held suburb of Damascus – as well as deep divisions and mistrust dividing Washington and Moscow.

The complexities have also grown with the increasing internationalization of what has largely become a proxy war between regional and world powers, highlighted by a move by Turkish troops across the Syrian border against Islamic State fighters this week.

Kerry said he and Lavrov had agreed on the “vast majority” of technical discussions on steps to reinstate a cease-fire and improve humanitarian access.

But critical sticking points remain unresolved and experts will remain in Geneva with an eye toward finalizing those in the coming days, he said.