Lithuanian border guards have uncovered another underground tunnel allegedly intended to smuggle migrants from Belarus into the European Union, authorities said Wednesday, marking the latest discovery along the two countries' heavily guarded frontier.

The unfinished, secret passageway, which had no exit, had been dug beneath the border fence at a depth of around 1.5 meters and reinforced with tree trunks and planks, they said.

Lithuania has repeatedly accused Belarus of organizing the smuggling of migrants from crisis-hit regions to the EU's external border. The government in Vilnius has responded by stepping up border security and erecting a border fence.

According to reports, the secret tunnel, with a camouflaged entrance, extended approximately four metres from Belarusian territory into Lithuanian territory. The border guards reportedly found the tunnel with the aid of electronic equipment.

Two other tunnels had already been discovered beneath the border between Belarus and Lithuania in recent weeks.

According to a media report, Lithuanian border guards were attacked in one instance by a group of migrants who were resisting detention. The officers were reportedly forced to withdraw, whilst the illegal border crossers fled back to Belarus.

Lithuania's Interior Minister Martynas Katelynas ordered an internal investigation after a spokeswoman for the border guard service confirmed the incident, which the authorities had not previously made public. Prime Minister Mindaugas Sinkevičius also criticized the delayed announcement of the attack on the border guards.