KHARTOUM (Reuters) - Sudan's inflation in May reached 60.93 percent, its highest level in years, the state statistics agency said on Thursday, as the country faces a worsening economic crisis. Inflation in April was 57.65 percent, it said. It said the cause of the May rise was because of higher food prices during increased demand for the Muslim holy month of Ramadan. Rising food prices have kindled unrest, notably with riots in January, and a hard currency shortage has crimped imports in recent months. The economy took a downturn after the United States lifted 20 years of sanctions on Khartoum. The Sudanese government rejected an IMF suggestion that it float its pound currency at the time, but it plummeted to record lows on the black market this year, prompting the central bank to make two steep devaluations. The official rate has reached about 31.5 pounds to the dollar from 6.7 pounds in late December. The Sudanese pound hit a record low of about 40 pounds to the dollar on the black market in April, but the devaluations and a ban on deposits of dollars obtained from the black market have reversed this trend. Sudan's central bank has also introduced restrictions on withdrawals, leaving many unable to extract their cash from banks. The African country has tried in vain to boost foreign investment since the lifting of sanctions, although it remains on the U.S. list of state sponsors of terrorism, restricting any potential U.S. aid to Sudan. Khartoum said last month it had severed all defence ties with North Korea, a rare admission that it used to have those ties in the first place. (Reporting by Khalid Abdelaziz, Writing by John Davison in Cairo; Editing by Alison Williams)