Ethiopia has been strengthening its military defences along the border with Eritrea, amid fears of a new conflict with its neighbour.
The UN says Eritrean restrictions are hindering its peacekeeping work
A BBC correspondent at the border says the Ethiopian army has been digging long lines of trenches and bunkers.
The United Nations has warned the situation is potentially volatile.
The two countries were at war from 1998 to 2000, leading to the loss of tens of thousands of lives. A long-running border dispute has still to be settled.
A few hundred metres from the border between Ethiopia and Eritrea, at Zalambassa, there are long lines of newly dug Ethiopian trenches and bunkers, reports the BBC's Peter Biles.
According to the army, it has all been constructed during the course of this year.
On Wednesday, Ethiopian Prime Minister Meles Zenawi confirmed that he had moved thousands of extra troops to the north of the country, saying it was to prevent an invasion by Eritrea.
Ethiopia is estimated to have nearly half its armoured units in the area. Eritrea has sent troops into the demilitarised zone on the border.
Ethiopia insists it will not strike first, and that the build-up of forces is purely defensive.
The two sides are separated by a UN-administered security zone, but in the past month Eritrea has placed severe restrictions on the movement of the UN peacekeepers.
UN troops say Eritrean restrictions on patrols and helicopter flights mean 60% of the border cannot be monitored.
The border town of Zalambassa was almost completely destroyed in the last round of fighting five years ago.
A major reconstruction programme has given it a new lease of life, but local people are now nervous about the prospect of another war, our correspondent says.