BERLIN — German Chancellor Angela Merkel on Saturday warned Moscow not to doubt Europe’s willingness to impose tough economic sanctions if Russia further violates Ukraine’s borders.

“If the territorial integrity of Ukraine continues to be violated, then we will have to introduce economic sanctions,” Merkel told a congress of her Christian Democratic Union (CDU) as it kicked off its campaign for the European Parliament elections next month.

“No one should doubt” Europe’s ability to agree to harsher measures, said Merkel, the leader of the region’s largest economy who has been key to shaping its response to the Ukraine crisis.

“Might doesn’t make right,” she added.

Kiev on Saturday accused Moscow of using “economic aggression” after the energy-rich giant hiked its gas price for Ukraine by 81 percent last week to the highest rate of any of its European clients.

The budding gas war adds another layer of tensions to the growing crisis as Russia has massed tens of thousands of troops along Ukraine’s eastern border after annexing its Black Sea peninsula of Crimea last month.

It has also inflamed the diplomatic row between Moscow and the West that has seen Kremlin insiders hit with sanctions and left Russia more isolated than at any stage since the 1989 fall of the Berlin Wall.

Russia needs to understand it is not in its interest “to have a collapsing state in its neighborhood,” German Foreign Minister Frank-Walter Steinmeier said Saturday at the close of two-day talks with his 27 European Union counterparts.

Meanwhile Ukraine’s security service said on Saturday it had arrested 15 men for allegedly plotting to stage an armed revolt in an eastern region neighboring Russia.

It added it had also seized 300 machineguns, a rocket launcher and numerous grenades from the suspected coup plotter.

The Ukrainian Security Service said the men planned to seize the administration building in the Lugansk region on Thursday, “frighten the local population, and use arms and explosives”.

The security agency provided no other details and did not reveal any political affiliation of the suspects.

The heavily Russified eastern regions of Ukraine have been hit by protests since last month’s overthrow in Kiev of a pro-Kremlin regime by leaders who are seeking a closer alliance with the West.

Some eastern regions have also sought to stage their own independence referendums similar to the one that resulted in Crimea being annexed by Russia.