South Korea fires warning shots at 'N Korea drone' over DMZ

South Korean soldiers have fired warning shots at a suspected North Korean drone flown across the heavily fortified border.

Yonhap news agency cited officials saying that soldiers fired about 20 rounds before the craft turned back.

Earlier, South Korea's president urged China to impose the strongest possible sanctions against North Korea, following its apparent nuclear test.

Pyongyang claims it has tested a hydrogen bomb.

That claim is doubted by experts, who say the blast, though probably nuclear, was not big enough to have been a thermonuclear explosion.