SINGAPORE, June 9 (Reuters) - China has issued its second batch of fuel export quotas for this year, totalling 18 million metric tons, with overall levels largely steady year on year despite existing export restrictions, four trade sources said.

Since March, China has curtailed outbound shipments to regions other than Hong Kong to safeguard domestic supply in the face of a protracted conflict in the Middle East that is disrupting oil flows.

Out of the total, some 13 million tons of quotas were allotted for exports of gasoline, diesel and jet fuel, while 5 million tons were for marine fuel oil, the sources said.

Sinopec, CNPC and Sinochem were awarded a total of 4.06 million tons of quota under "processing trade category", which will be mainly for exports to Hong Kong and for aviation fuel refuelling at Chinese airports, the sources added.

Beijing in December allotted 19 million tons for the first batch of quotas for this year, with the second batch last year at 18 million tons.

China's commerce ministry was not immediately available outside of office hours.

The government also issued under "general trade" category 8.94 million tons of quota to six companies, but under the current restrictions only Sinopec and PetroChina were allowed to export, two of the four sources said.

Exports to ex-Hong Kong buyers, mostly in the Asia-Pacific region, were estimated at 500,000-550,000 tons for May and June.

The government vetted the list of countries receiving Chinese fuel during the past few months, but is relaxing the scrutiny for June shipments, said two separate sources with knowledge of the matter.

(Reporting by Siyi Liu, Trixie Yap and Chen Aizhu; Editing by Alison Williams)