BEIRUT, Lebanon — President Bashar al-Assad of Syria on Sunday banned the use of foreign currencies in business transactions, part of the government’s effort to prop up the Syrian pound, which has plummeted to 200 pounds to the dollar from 47 to the dollar in March 2011.

SANA, the government-run news agency, said that people offering goods and services for foreign currencies without the government’s approval could be fined and sentenced to at least six months in jail. In cases involving deals valued at more than $5,000, the punishment could be up to 10 years of hard labor.

Syrians have grown increasingly desperate to move their money to foreign currencies as the civil war, now in its third year, ravages the economy. Currency exchange shops in central Damascus are often packed. Real estate and manufacturing deals are increasingly conducted in dollars. Even taxi drivers and others working in small, informal businesses often demand to be paid in foreign currencies.

Last month, the Central Bank was able to reverse a deeper plunge in the value of the pound, which briefly traded at more than 300 to the dollar, by selling off dollars at a deep discount to banks, said Steve H. Hanke, a professor of applied economics at Johns Hopkins University who studies troubled currencies. The banks then sold the dollars to the public for a modest profit.