Eritrea has imposed fuel rationing - although there has been no official announcement of the move which began about two weeks ago.
Petrol was also restricted last year
Diplomats, who asked not to be named, say they are being limited in the amount of petrol and diesel they are allowed each week.
"We are allowed, per week and per car, 30 litres of petrol and 40 litres of diesel," a western diplomat told the AFP news agency.
Taxis are reported to have increased their fares, but there are no queues at petrol stations.
Eritrea is short of foreign exchange after buying large quantities of modern aircraft to use in the war it fought with Ethiopia that ended four years ago.
The border between Ethiopia and Eritrea has been closed ever since, depriving Eritrea of access to its natural markets.
Fuel rations were imposed for about a month last year.
A full explanation for last year's introduction of coupons was not given.
But a government spokesman did say that there were other more important things to spend hard currency on, such as bread for poor people.