By John Irish and Warren Strobel
MUNICH (Reuters) - Major powers agreed on Friday to a "cessation of hostilities" in Syria to begin in a week, doing nothing in the meantime to halt Russian bombing poised to give government forces their biggest victory of the five-year-old war.
If implemented, the deal would allow humanitarian aid to reach besieged towns. It has the potential to be the first diplomatic breakthrough in a conflict that has fractured the Middle East, killed at least 250,000 people, made 11 million homeless and sent hundreds of thousands fleeing into Europe.
But by allowing fighting to rage on for at least another week, it gives the Damascus government and its Russian allies time to press on with an offensive that has transformed the conflict since the start of this month.
Russian warplanes were bombing northern Syria on Friday, showing no sign of slowing the pace of attacks despite the agreement hammered out overnight.
Syrian government forces, with Lebanese and Iranian allies and vigorous Russian air support, are now poised to recapture the northern city of Aleppo, Syria's biggest city before the war, and seal off the border with Turkey.
Those two victories would reverse years of insurgent gains and effectively end rebel hopes of dislodging President Bashar al-Assad through force, the cause they have fought for since 2011 with the encouragement of Arab states, Turkey and the West.
The "cessation of hostilities" agreement reached by the world powers falls short of a formal ceasefire, since it was not signed by the main warring parties - the opposition and government forces.
It was announced after marathon talks in Munich aimed at resurrecting Geneva peace talks that collapsed last week. Implementation will now be the key, said U.S. Secretary of State John Kerry: "What we need to see in the next few days are actions on the ground, in the field."
Russia made clear from the outset that the "cessation" would not apply to its air strikes, which have decisively shifted the balance of power toward its ally President Bashar al-Assad since Moscow joined the conflict four months ago.
Foreign Minister Sergei Lavrov told the news conference that Moscow would not stop bombing, as the deal did not apply to Islamic State and to a rebel group called the Nusra Front, which is affiliated with al Qaeda.
"Our airspace forces will continue working against these organizations," he said.
Moscow has always said that those two jihadist groups are the only targets of its air campaign. Western countries say Russia has in fact been mostly targeting other insurgent groups, including some that they support.
NATO Secretary General Jens Stoltenberg said Moscow must halt strikes on insurgents other than Islamic State for any peace deal to work.
"Russia has mainly targeted opposition groups and not ISIL (Islamic State). Air strikes of Russian planes against different opposition groups in Syria have actually undermined the efforts to reach a negotiated, peaceful solution," he said.
The United States has been leading a separate air campaign against Islamic State fighters since 2014, when that group, also known as ISIS or ISIL, swept through much of eastern Syria and northern Iraq, declaring a caliphate.
Washington has largely steered clear of intervening in the main battlefields of Syria's civil war in the west of the country, leaving the field to Russia which began its air campaign on Sept. 30 last year.
Kerry had entered the Munich talks pushing for a rapid halt in the conflict, with Western officials saying Moscow was holding out for a delay.
The tactic of agreeing to a future break in hostilities, while continuing to fight for gains on the ground, is one Moscow's allies used in eastern Ukraine only a year ago.
A truce there eventually took hold, but only after Russian-backed separatist fighters had achieved a major victory, overrunning a besieged town in a final offensive after the deal was reached.
Diplomats from countries backing the plan were due to meet on Friday to discuss sending in urgent humanitarian aid.
"We have high hopes that the parties in the International Syria Support Group, including Russia and the United States, will do everything they can to push for humanitarian access to civilians in need inside Syria," said Jan Egeland, the head of the Norwegian Refugee Council, who will chair the meeting.
"This could be the breakthrough we have been waiting for to get full access to desperate civilians inside Syria. But it requires that all those with influence on all sides of the conflict are putting pressure the parties."
The Syrian government has for years repeatedly made promises of humanitarian access but has rarely lived up to them. Western-backed rebels have also been accused of obstruction of aid.
The sides meeting in Munich also called for a resumption of political peace talks, which collapsed last week before they began after the opposition demanded a halt to bombardment.
The powers reaffirmed long-standing commitments to a "political transition" in Syria once conditions allow. Kerry said: "Without a political transition, it is not possible to achieve peace."
There has been disagreement for years over whether "transition" requires Assad to leave power, as Western countries have been demanding in vain since 2011.
The Syrian leader says he will not go, and his Russian and Iranian allies say it must be up to Syrians to decide his future, a position seen as supporting elections which Assad would be expected to win.
Lavrov said political peace talks should resume in Geneva as soon as possible and that all Syrian opposition groups should participate. Moscow has complained in the past that Kurdish groups were excluded under pressure from Turkey.
Syria's main opposition group cautiously welcomed the plan, but said it would not agree to join political talks unless the agreement proved effective.
A senior French diplomat said: "The Russians said they will continue bombing the terrorists. They are taking a political risk because they are accepting a negotiation in which they are committing to a cessation of hostilities.
"If in a week there is no change because of their bombing, then they will bear the responsibility."
In Syria, rebels said the town of Tal Rifaat in northern Aleppo province was the target of intensive bombing by Russian planes on Friday morning.
The British-based Syrian Observatory for Human Rights said warplanes believed to be Russian also attacked towns in northern Homs.
(Additional reporting by Denis Dyomkin, Shadia Nasralla and Robin Emmott in Munich, and Suleiman al-Khalidi in Amman; writing by Peter Graff; editing by Andrew Roche)