By Laurence Norman 

BRUSSELS--European Union leaders agreed late Saturday to draw up options within a week for possible new sanctions against Russia, with action to follow quickly unless Moscow takes clear steps to scale back its intervention in Ukraine.

European Council President Herman Van Rompuy said the bloc wouldn't set out specific criteria for triggering fresh sanctions but said there was "determination" to ensure Russia paid an appropriate price for heightening tensions.

"I can assure you that everyone is fully aware that we have to act quickly given the escalation on the ground," he said at the end of a summit of European leaders.

In a statement, EU leaders said the bloc "stands ready to take significant further steps in light of the situation on the ground." They also requested the Commission to present an option to slap sanctions on "every person and institution" dealing with rebel groups in the Donbas region of eastern Ukraine.

A fresh move to tighten sanctions against Russia will fuel tensions between Brussels and Moscow and could lead to retaliation from Russia, which denies it has sent troops into Ukraine. Earlier this month, Russia banned imports on a wide range of food and agriculture products from the EU in response to Western sanctions.

Some in the EU, whose economy is still in a fragile recovery after a long financial and debt crisis, have also warned that tougher sanctions could make it harder for the bloc to find a political solution to the crisis.

There was no immediate reaction to Saturday's EU decision from Russian officials in Brussels.

Until 72 hours ago, EU leaders weren't expected to take any fresh action against Russia this weekend following Brussels-brokered talks in Minsk on Tuesday between Russian President Vladimir Putin and his Ukrainian counterpart, Petro Poroshenko.

The mood soured significantly as reports emerged that hundreds of Russian soldiers had entered Ukraine.

With pro-Moscow rebels making fresh gains on the ground, a host of EU leaders said Saturday the deteriorating situation in eastern Ukraine necessitated an EU reaction.

German Chancellor Angela Merkel said sanctions were inevitable if the situation doesn't improve significantly. "Should the situation stay as we see it today, or should there be further escalation, sanction in the areas the commission has looked at up until this point, such as on the energy and financial sector, will be heightened."

U.K. Prime Minister David Cameron said it is "totally unacceptable" that there are Russian soldiers in Ukraine "and we have now set out a timetable for further sanctions...and that's important."

French President François Hollande said there would have to be another meeting either of senior EU officials to sign off any new measures. Both he and Ms. Merkel talked down the prospect of the bloc meeting Ukraine's request to send military equipment to Kiev.

"One can't think it would be fruitful, useful," Mr. Hollande said. "What we're asking for today is a cease-fire--not to sustain the conflict, but to end it."

Mr. Poroshenko, who also attended Saturday's gathering in Brussels, announced his government was working on a peace plan but warned that the Ukraine crisis could soon reach "the point of no return."

President Barack Obama has ruled out a U.S. military response but said Thursday that Russia's escalating incursion into Ukraine would bring more costs for Moscow. He said further actions would be discussed at a NATO summit next week.

In July, the EU for the first time agreed broad sanctions against Russia's financial, energy and defense sectors, the sharpest steps since Moscow annexed Crimea in March.

Still, there were plenty of exemptions to the restrictions. Member states were allowed to complete already signed defense contracts. The financial measures allowed most transactions to continue and only placed significant restrictions on five state-owned Russian banks.

The U.S. followed the EU's moves in July by adopting similar sanctions against Russian banks as well as the energy, arms and shipping sectors.

The EU has significant energy, financial and commercial links with Russia and some member states have been cautious about heightening tensions with Moscow.

Austrian Chancellor Werner Faymann was one of several leaders Saturday to question whether sanctions would have any impact on Mr. Putin. In the past Mr. Putin has appeared to exploit those divisions, taking steps to temporarily ease tensions to avoid tougher sanctions.

On Saturday, as fresh news came from Kiev of rebel gains on the ground, Mr. Poroshenko said he hopes the peace plan that's being drafted can form the basis of discussions Monday on a cease-fire between Russia and Ukraine. He said he still would like to see a political solution to the escalating crisis.

Russia, which says a group of soldiers arrested by Ukrainian authorities had entered the country by accident, has repeatedly called on Kiev to end its offensive against the rebels. Ukraine and its western allies say Moscow must offer guarantees to stem flows of fighters and weapons across its border for a sustainable cease-fire to work.

Greg White in Moscow, Sam Schechner in Paris and Frances Robinson in Brussels contributed to this article. Greg White in Moscow, Sam Schechner in Paris and Frances Robinson in Brussels contributed to this article.

Write to Laurence Norman at laurence.norman@wsj.com