Obama Asks Putin to Help Halt Assad Offensive in Syria

President Barack Obama spoke to Russian President Vladimir Putin tonight to urge him to use Russia's influence to halt escalating fighting in both Syria and Ukraine, the White House said in a statement.

Shelling has been increasing along the front line in eastern Ukraine, which has laid largely dormant since last year, while the partial cease-fire in Syria negotiated by Moscow and Washington, D.C., in February tonight appeared to be on the verge of final collapse.

The Syrian opposition announced it was suspending its participation in United Nations-brokered talks in Geneva, while rebel groups inside Syria declared a major offensive against government forces in retaliation for continuing airstrikes. The announcements potentially herald an end to the partial truce that had dramatically reduced violence in Syria and threaten to definitively torpedo the Geneva talks, seen as the best hope of ending the five-year conflict.

The opposition blamed the renewed violence on offensives by troops loyal to Syrian President Bashar al-Assad, whom they've accused of pounding areas under rebel control and of launching an advance against the city of Aleppo. A representative for the main opposition negotiating group, the High Negotiations Committee (HNC), which is backed by the U.S., said talks could not continue while government troops conducted major attacks on the opposition.

A White House statement said Obama had stressed to Putin the need to persuade his ally Assad to halt offensive operations to preserve the truce Moscow helped establish in February. The White House hopes Moscow, as Assad's key military ally, can pressure the Syrian leader to abide by the cease-fire.

In a statement, the Kremlin said Putin reiterated his commitment to preserving the partial cease-fire, but added that the moderate opposition must "distance themselves swiftly from ISIS and Jabhat al-Nusra," an al-Qaeda affiliate. Neither group is included in the cease-fire agreement and the U.S. is concerned that Assad and Moscow are using the exception as an excuse to attack moderate groups, as they have done previously.

Although Russia has curtailed its operations against rebels since a partial drawdown of its air presence in Syria last month, it continues to supply Assad with weapons and fly sorties against groups it claims to be outside the cease-fire. Some opposition groups have accused Russian aircraft of supporting the renewed Assad assaults against the opposition, though Moscow has denied this.

Last week, Putin blamed renewed violence around Aleppo on rebels.

Obama also asked Putin to take steps to help end what the White House statement called the "significant uptick" in violence in eastern Ukraine, where shelling between pro-Russian rebels and Ukrainian troops has been steadily intensifying in recent weeks.

Putin expressed his hope that Ukraine's new government would make efforts to fulfill its obligations under a peace plan brokered last year.

The phone call also followed a diplomatic spat between Moscow and Washington last week, after Russian warplanes repeatedly buzzed an American destroyer in the Baltic Sea, prompting loud protests from the Pentagon. Moscow has denied its aircraft did anything inappropriate.

Despite the vociferous complaints from U.S. officials, a White House spokesman said Obama did not raise the incident with Putin.