MOSCOW — Russia filed a lawsuit against Ukraine in the London High Court on Wednesday over $3 billion in unpaid loans, escalating a legal battle that is adding to the geopolitical tensions between the two countries.

The lawsuit centers on a controversial $3 billion loan that Russia granted to Ukraine in 2013 under the former Ukrainian president. In December, Ukraine declared a moratorium on paying the debts, unless Russia joined a restructuring program that included other creditors.

The dispute is the latest between Ukraine and Russia to spill off the battlefield and into European courts. While a shaky cease-fire has reduced violence in eastern Ukraine, a year-old peace accord still faces major obstacles.

The new Ukrainian cabinet has insisted that Moscow agree to the same 20 percent write-down and delay in repayment that was accepted by commercial creditors in a restructuring deal last year. President Vladimir V. Putin of Russia has rejected the deal as unfair.