KYIV, Jan 18 (Reuters) - Romanian farmers blocked a crossing on the Romanian-Ukrainian border on Thursday, Ukraine's state customs service said.

Farmers and hauliers have been blocking highways and slowing traffic with convoys of tractors and trucks in protest against the high cost of diesel, insurance rates, European Union measures to protect the environment and pressures on the domestic market from imported Ukrainian agricultural goods.

The protesters want a moratorium on loan repayments, faster subsidy payments and separate lines at border crossings and the Black Sea port of Constanta for EU lorries and trucks from outside the bloc, including Ukraine.

"The blockage is being carried out by large agricultural machinery," the Ukrainian state customs service said of Thursday's blockade of Halmeu-Diakove crossing.

"Traffic leaving Ukraine for trucks has been temporarily suspended to prevent passenger cars from being blocked. The duration of the blockade is not yet known," it said on the Telegram messaging app.

Romanian Black Sea ports are key for Ukrainian agricultural exports amid Russia's partial blockade of the Black Sea and logistical difficulties in transit through Ukrainian land border with the EU.

Until a deal was reached this week, Polish hauliers had also been blockading some Ukrainian border crossings in protest at what they said was unfair competition from their Ukrainian counterparts. The protests pressed demands that the EU reinstate a system in which Ukrainian companies need permits to operate in the bloc and the same for European truckers entering Ukraine. (Reporting by Pavel Polityuk, Editing by Timothy Heritage)