President Barack Obama and German Chancellor Angela Merkel on Monday warned of additional steps in Ukraine should diplomacy fail to deter Russian action in the country.

“Russian aggression has only reinforced the unity of the United States and Germany and our allies and partners around the world,” Obama said during a joint press conference with Merkel.

The U.S., Germany and other European partners have imposed successive waves of sanctions on Russia for what they say are the Kremlin’s destabilizing actions in Ukraine, including the annexation of Crimea in March 2014, and the continued arming, training, and reinforcement of separatist rebels in eastern Ukraine.

“If we give up this principle of territorial integrity of countries, then we will not be able to maintain the peaceful order of Europe that we’ve been able to achieve. This not just any old point: it’s an essential, a crucial point, and we have to stand by it,” Merkel said.

The American president added that should diplomacy fail to deter Russia, he has directed his policy team to look at all options, “and the possibility of lethal defensive weapons is one of those options that’s being examined.”

Earlier Monday, the Ukrainian military said 1,500 Russian troops crossed its border with convoys of military hardware, including Grad rocket systems.

Obama has faced strong pressure from Congress to supply the Ukrainian military with lethal assistance so it can more effectively fight separatist rebels in the east of the country.

“What we’re doing with regard to Ukraine and with regard to Russia makes no sense, and it isn’t working,” prospective Republican presidential candidate Sen. Ted Cruz told CNN on Sunday. “It is long past time for us to step forward and provide defensive weapons, so that the men and women of Ukraine can defend their nation.”

The German chancellor has argued publicly against such a move, in part due to fears Russia could increase its military activity in Ukraine. She has jointly planned a peace summit with French President Francois Hollande scheduled for Wednesday in the Belarusian capital, Minsk.

Ukrainian President Petro Poroshenko and Russian President Vladimir Putin are slated to participate in the French-German meeting.

Obama said he “won’t prejudge” the talks, but earlier said Russia has “violated just about every commitment they made” during a previous peace conference in the Belarusian capital.

Despite her continued emphasis on diplomacy, Merkel said “if, at a certain point in time, one has to say that a success is not possible, even if one puts every effort into it, then the United States and Europe have to sit together and try and explore further possibilities.”

More than 5,300 people have died in ongoing fighting between the central government in Kiev and separatist rebels, according to a UN tally.

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