SEOUL - South Korean police have banned former prime minister Han Duck-soo and ex-finance minister Choi Sang-mok from travelling abroad as part of a probe into alleged insurrection linked to former leader Yoon Suk-yeol's martial law bid, a police official said.

The travel ban was imposed in mid-May, the Yonhap news agency reported. Han and Choi were questioned on Monday by a special police unit investigating former top government officials over insurrection charges, Yonhap said.

ALSO READ: S. Korea ex-President Yoon leaves conservative party

After Yoon was impeached for violating the duties of his office in December, there have been questions about what role Han and Choi played in the former leader's short-lived martial law and if they had resisted the move as they claimed.

The December 3 martial law declaration, which was revoked hours later by parliament, thrust South Korea into an unprecedented constitutional crisis that involved both Han and Choi serving as acting president.

READ MORE: S. Korea's Democratic Party candidate leads poll for June 3 presidential election

South Koreans vote in a snap election on June 3 to elect a new leader.