South Korea summoned the Russian ambassador 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 powerful sister of North Korean leader Kim Jong Un issued a vague threat of retaliation after South Korean activists flew balloons carrying anti-Pyongyang propaganda leaflets across the border, and South Korea's military said it had fired warning shots the previous day to repel North Korean soldiers who briefly crossed the rivals' land border for the third time this month.