South Korea summoned the Russian Ambassador Georgy Zinoviev to protest the country’s new defense pact with North Korea on Friday, as border tensions continued to rise with vague threats and brief, seemingly accidental incursions by North Korean troops.

Earlier Friday, the sister of North Korean leader Kim Jong Un issued a vague threat of retaliation after South Korean activists flew balloons carrying anti-Pyongyang propaganda across the border, and the South said it fired warning shots to repel North Korean soldiers for the third time.