ISLAMABA, June 11 (Reuters) - Pakistan's Prime Minister Shehbaz Sharif on Sunday said the first cargo of discounted Russian crude oil arranged under a new deal struck between Islamabad and Moscow had arrived in Karachi.

"Glad to announce that the first Russian discounted crude oil cargo has arrived in Karachi and will begin oil discharge tomorrow," Sharif tweeted.

"This is the first ever Russian oil cargo to Pakistan and the beginning of a new relationship between Pakistan and Russian Federation," he added.

A port official said on Sunday evening that the oil was in the process of being unloaded.

Reuters first reported on the deal in April. The discounted crude offers a relief to Pakistan, which is facing a payments crisis and is at risk of defaulting on its debt.

Pakistan's purchase gives Russia a new outlet, adding to Moscow's growing sales to India and China, as it redirects oil from western markets because of the Ukraine conflict.

Energy imports make up the majority of Pakistan's external payments. The country's imports of crude are expected to reach 100,000 barrels per day after the first cargo arrives on Monday.

There has been no confirmation of how payment would be made, but Pakistan recently announced a plan to allow barter trade with Russia, Afghanistan and Iran, which analysts said could reduce the need for dollars and the risk of cross-border smuggling of energy products.