DUBAI, May 29 (Reuters) - Foreign assets at Saudi Arabia's central bank shrank $6 billion in April as it liquidated some financial holdings to cover a big state budget deficit caused by low oil prices, official data showed on Sunday.

Net foreign assets dropped 1.1 percent from the previous month to 2.15 trillion riyals ($572 billion). Assets fell 15.7 percent from a year earlier to their lowest level since April 2012; they reached a record high of $737 billion in August 2014 before starting to shrink.

The foreign assets are mainly denominated in U.S. dollars, in the form of securities such as U.S. Treasury bonds and deposits with banks abroad, fund industry sources told Reuters.

Earlier this month, the U.S. Treasury disclosed the size of Saudi Arabia's U.S. Treasury holdings for the first time; it said the kingdom owned $116.8 billion of Treasuries in March.

Deposits with banks abroad fell 2.2 percent from the previous month to $129 billion in April, while investment in foreign securities edged down 0.8 percent to $386 billion.

(Reporting by Andrew Torchia; Editing by Andrew Bolton)