GENEVA (Reuters) - The United Nations Syria mediator called on warring sides there to prepare to engage in substantive political discussions at the next round of talks starting on Thursday, a spokeswoman said on Tuesday.

Rebel fighters and their families that evacuated the besieged Waer district in the central Syrian city of Homs, after an agreement reached between rebels and Syria's army, arrive on the southern outskirts of the Syrian city of al-Bab, Syria March 19, 2017. REUTERS/Khalil Ashawi

The talks - expected to last until April 1 - will focus on governance, constitutional change, U.N.-sponsored elections and counter-terrorism measures, U.N. spokeswoman Alessandra Vellucci said in a statement read out at a U.N. briefing.

Syrian government and opposition delegations to the fourth round last month have confirmed their attendance, she said.

De Mistura has urged all sides “to do the necessary preparations in advance of the fifth round to be ready to engage substantively and proactively on the substance of all four baskets on the agenda in parallel”, she added.

The U.N. envoy, who has just held talks in Riyadh, is conducting intensive shuttle diplomacy between the backers of opposing sides this week. He is going to consult officials in Moscow and Ankara before returning to Geneva a day after the talks open under his deputy Ramzy Ramzy.

But difficulties facing Russian-led efforts to revive Syria diplomacy surfaced last week when Turkish-backed rebel groups stayed away from ceasefire talks in Kazakhstan, criticizing Moscow for failing to get the Syrian government and its militia allies to adhere to a truce brokered in late December.

The war, now in its seventh year, rages on as President Bashar al-Assad seeks to press his advantage by inflicting total defeat on his opponents, supported by the Russian military and Iranian-backed militias.

Government forces are pressing an offensive to defeat rebels in their last major stronghold near Damascus, the Eastern Ghouta. Rebels have mounted a counter attack on the northeastern periphery of the capital.

Air strikes have also escalated against insurgent-held areas west of Aleppo and in Idlib province, according to the Syrian Observatory for Human Rights.

Syrian rebels stormed a government-held area in northeastern Damascus on Tuesday for the second time in three days, sources on both sides said, pressing the boldest assault on the capital by opposition fighters in several years.