WRAPUP 2-Group of Seven agrees swift sanctions against Kremlin over Ukraine

(Removes reference to any G7 meeting in third and fourth paragraph)

* Washington could unveil its sanctions as early as Monday

* They will target Kremlin "cronies" - U.S. officials

* International observers held in separatist-controlled town

* Separatists accuse observers of spying for NATO

By Thomas Grove and Matt Spetalnick

SLAVIANSK, Ukraine/SEOUL, April 26 (Reuters) - Leaders of the Group of Seven major economies agreed to impose extra sanctions on Russia over its intervention in Ukraine, where armed pro-Moscow separatists detained a group of international observers and accused them of being NATO spies.

The United States said its part of the new punitive measures, which U.S. officials said would target "cronies" of Russian President Vladimir Putin, could be unveiled as early as Monday unless Russia moved fast to defuse the Ukraine crisis.

In a joint statement, the G7 leaders said Russia had not taken any concrete steps to implement an accord, signed in Geneva, that was intended to rein in illegal armed groups.

"Instead, it has continued to escalate tensions by increasingly concerning rhetoric and ongoing threatening military manoeuvres on Ukraine's border," it said.

"We have now agreed that we will move swiftly to impose additional sanctions on Russia ... We have committed to act urgently to intensify targeted sanctions and measures to increase the costs of Russia's actions."

But it added: "We underscore that the door remains open to a diplomatic resolution of this crisis."

Russia denies it is to blame for the crisis in eastern Ukraine, where armed pro-Russian separatists have taken control of about a dozen officials buildings are defying the rule of the Western-backed government in the capital, Kiev.

The Kremlin argues that the crisis began when a new leadership took over in Kiev, in what Moscow calls a coup d'etat, and started persecuting the Russian-speaking community in the east for wanting closer ties with Russia.

The crisis has brought relations between Russia and the West to their lowest level since the Cold War, and is increasingly turning into a military stand-off.

Russia has massed troops and helicopters on the border with Ukraine where it says they are conducting exercises, while NATO has deployed extra forces in eastern Europe, saying they are needed to reassure its allies.

OBSERVERS HELD

The international observers were being held in the eastern city of Slaviansk, a flashpoint between the Moscow-backed separatists who control the city, and Kiev's forces who are trying to squeeze them out.

They were part of a German-led monitoring mission visiting the area under the aegis of the Organization for Security and Cooperation in Europe (OSCE), a Vienna-based body whose 57 member states include Russia.