LONDON — Julian Assange, the founder of WikiLeaks, said on Monday that his controversial website could be forced to shut down by the end of the year because a 10-month-old “financial blockade” had sharply reduced the donations on which it depends.

Calling the blockade a “dangerous, oppressive and undemocratic” attack led by the United States, Assange said at a news conference that it had deprived his organization of “tens of millions of dollars,” and warned, “If WikiLeaks does not find a way to remove this blockade, we will not be able to continue by the turn of the new year.”

Since the end of 2010, financial intermediaries, including Visa, MasterCard, PayPal and Western Union, have refused to allow donations to WikiLeaks to flow through their systems, he said, blocking “95 percent” of the website’s revenue and leaving it to operate on its cash reserves for the past 10 months. An aide said that WikiLeaks was now receiving less than $10,000 a month in donations.