Brown faced journalists at a news conference a day after Labour's general secretary Peter Watt stepped down, acknowledging he was aware a key donor was channeling donations through associates to dodge transparency rules.
Businessman David Abrahams said he gave Labour 400,000 pounds (US$825,000; EUR556,000) through the accounts of others.
Brown said this was "completely unacceptable, cannot be justified and this behavior should never happen again in future."
Russia has delivered three divisions of anti-aircraft missile systems S-300PM-2 to Syria. These systems differ from the classic S-300