DUBAI, Sept 29 (Reuters) - Net foreign assets at Saudi Arabia's central bank fell to $554 billion in August, down by about $1 billion from July, as the government drew on reserves to cover a budget deficit caused by low oil prices, official data showed on Thursday.

Assets shrank by 15.4 percent from a year earlier to their lowest level since January 2012. They reached a record high of $737 billion in August 2014 before starting to fall.

The assets are believed to be mainly denominated in U.S. dollars, in the form of securities such as U.S. Treasury bonds and deposits with banks abroad.

Those deposits shrank 4.2 percent from the previous month to $119 billion in August, but holdings of foreign securities rose 1.5 percent to $376 billion.

The government has also been borrowing domestically and abroad to cover part of its deficit, which totalled nearly $100 billion last year. Bankers expect it to conduct its first international bond issue to raise about $10 billion or more by the end of October.

(Reporting by Andrew Torchia; Editing by Toby Chopra) ((andrew.torchia@thomsonreuters.com; +9715 6681 7277; Reuters Messaging: andrew.torchia.thomsonreuters.com@reuters.net))