The offensive began early in the morning, using planes and helicopter gunships to bombard Eritrean positions about 70 kilometres west of the port city. Heavy shelling reportedly continued throughout the day.
An M1-24 helicopter was shot down by Eritrean forces at 9am, said the Eritrean government. The presidential adviser Yemane Gebremeskel said identity cards from the dead crew had been retrieved. He said no troop movements had been reported, but described the attack as an attempt to "soften the front line" for further attack.
The Red Sea port of Assab is crucial to land-locked Ethiopia - which, until the conflict began last May, provided 90 per cent of all transactions in the port. When Eritrea won independence from Ethiopia in 1993, Ethiopia secured access to the Red Sea through bilateral agreement and co-operation.
Since the conflict, Ethiopia has resorted to using the port of neighbouring Djibuti.
Unlike the border dispute over the Badme triangle - which triggered the conflict - there is no territorial claim over Assab by Ethiopia.