UNITED NATIONS, July 12 (Reuters) - The United States accused North Korea on Thursday of breaching a U.N. sanctions cap on refined petroleum by making illicit transfers between ships at sea, according to a document seen by Reuters and which Washington submitted to Security Council members."Our information suggests that between January 1 and May 30 of 2018, DPRK (North Korean) tankers have called in port in the DPRK at least 89 times likely to deliver refined petroleum products illicitly procured via STS (ship-to-ship) transfers," the United States told the U.N. Security Council's North Korea sanctions committee.The United States did not say which countries it believed were illicitly providing North Korea with refined petroleum.In December the 15-member Security Council capped refined petroleum product exports to North Korea at 500,000 barrels a year. The U.S. accusation of a sanctions breach comes as Washington engages with North Korea in a bid to negotiate denuclearizing the Asian country.According to the Security Council North Korea sanctions committee website, only Russia and China have reported legitimate sales of some 14,000 tons of refined petroleum to North Korea in 2018."These sales and any other transfer must immediately stop since the United States believes the DPRK has breached the ... refined petroleum products quota for 2018," the United States said in a document submitted to the committee.The United States provided a list to the Security Council committee of the 89 North Korean tankers and a few select photos, seen by Reuters."If fully loaded at around 90 percent laden, DPRK tankers have delivered nearly triple the 2018 quota at 1,367,628 barrels," the United States said.It asked the North Korea sanctions committee to issue an urgent note to all U.N. member states notifying them that North Korea has breached the refined petroleum cap and order an immediate halt to all transfers.The U.N. Security Council has unanimously boosted sanctions on North Korea since 2006 in a bid to choke off funding for Pyongyang's nuclear and ballistic missile programs, banning exports including coal, iron, lead, textiles and seafood, and capping imports of crude oil and refined petroleum products.In March the council blacklisted dozens of ships and shipping companies over oil and coal smuggling by North Korea.