South Korea’s military says it has fired warning shots at an unidentified object flying south from rival North Korea.

The military said in a statement Tuesday that it has bolstered its air surveillance after the incident but provided few other details.

Yonhap news agency reported, without citing a source, that South Korea fired about 90 machine gun rounds into the air and toward North Korea. It said South Korea was analyzing whether a North Korean drone had crossed the border.

The Koreas face off across the world’s most heavily armed border, and the two sides occasionally clash.

Attacks blamed on North Korea in 2010 killed 50 South Koreans.