By Benoît Faucon 

LONDON--Libya said Wednesday it was resuming oil exports from its largest terminal after a year-long stoppage, as its oil industry manages to recover despite intense fighting in large cities.

But new uncertainty emerged later in the day as Libya's de facto oil minister said he was being replaced.

Italian tanker Maria Bottiglieri, which arrived overnight, is set to leave later Wednesday from the Es Sider terminal with about 600,000 barrels on board, Mohamed el-Harari, a spokesman for the state-owned National Oil Co. told The Wall Street Journal. Mr. el-Harari said the vessel was chartered by Austria's OMG AG and is bound for the Italian port of Trieste, confirming information from shipping and oil managers.

The terminal, which has an export capacity of 350,000 barrels a day, hasn't shipped any oil to international markets since July 2013, when it was occupied by an armed group seeking greater regional autonomy for Eastern Libya. But after a rebel attempt to sell oil was stopped by U.S. marines, the militants agreed to reopen the port--along with the neighboring Ras Lanuf terminal--last month.

While the first Es Sider export will draw on oil in storage at the terminal, production to supply the terminal--notably the 150,000-barrels-a-day Waha field--is set to restart mid-next week, Mr. el-Harari said.

Together with the resumption on Libya's largest oil field Sharara in the West of the country, the reopening of oil ports has now pushed production to about 560,000 barrels a day--nearly four times its level late May, he added.

Weary of the country's unreliability, oil buyers haven't rushed to purchase Libya's returning barrels. But after state-owned company National Oil Co. cut its prices, OMV and Unipec, the trading arm of China Petroleum & Chemical Corp., or Sinopec, have largely replaced traditional Libyan oil buyers from the Mediterranean and together bought the majority of the country's oil cargoes this month, according to a shipping broker. Sinopec didn't return a request for comment while OMV declined to comment.

The positive news was overshadowed by the resignation of Libya's de facto oil minister, Omar Shakmak, who had been at the helm of the oil sector in a caretaking capacity since January. Mr. Shakmak said he was leaving his official position as deputy oil minister at the request of Prime Minister Abdullah al-Thani. Mustafa Sanalla, chairman of the state-owned National Oil Co., will replace him for the time being, NOC officials said

Under Mr. Shakmak's watch, Libya's oil industry has staged a strong recovery in recent weeks despite intense clashes between rival militia in large cities, notably in the capital of Tripoli. Fighting escalated early Monday when, according to Libya's government website, aircraft bombed locations controlled by an Islamist militia that is trying to take over Tripoli's airport.

Write to Benoît Faucon at benoit.faucon@wsj.com