* Quasi-sovereign Saudi institutions investing more overseas
* Commercial bankers believe Public Investment Fund involved
* Foreign reserves increased moderately last month
* Authorities not imposing capital controls (Recasts with comments on foreign reserve levels, capital controls, context)
By Marwa Rashad and Stephen Kalin
RIYADH, Sept 16 (Reuters) - Foreign reserves at Saudi Arabia's central bank have been increasing this year and a large proportion of recent capital outflows has been due to foreign investment by other Saudi institutions, a senior central bank official said on Sunday.
Brent oil has jumped near $80 a barrel from $67 at the end of 2017, swelling Saudi Arabia's current account surplus and shrinking its state budget deficit; this has reduced the need to liquidate assets to finance government spending.
However, the central bank's net foreign assets -- a measure of the country's ability to defend its currency if needed -- have risen only marginally, to $493.8 billion at the end of July from $488.9 billion at the end of 2017.
Data for August has not yet been published, but Ayman bin Mohammed al-Sayari, deputy governor for investment, said the foreign reserves increased last month. Using a slightly different definition which includes gold and special drawing rights, he said the reserves rose to $509-510 billion at the end of August from $502 billion in July.
Sayari said Saudi institutional investors were coming to the central bank to exchange their local currency for hard currency that would be used to invest abroad.
This may explain the slow pace of increase in the central bank's foreign assets this year.
“A lot of the capital flows or at least a considerable portion of that...figure was merely some other institutional investors, quasi-sovereign, who have elected to...invest more internationally than locally," he told a news conference.
This pattern was seen in the first two quarters of the year, he said.
Sayari did not name the institutions, but many commercial bankers believe the Public Investment Fund, the kingdom's top sovereign wealth fund, is boosting its spending abroad.
The PIF's assets, estimated at over $250 billion, are mostly in the form of domestic equities and land. The fund has said it will raise the foreign portion of those assets substantially in order to boost returns and win access to technology and markets that benefit the Saudi economy.
For example, in late 2016 the PIF said it would invest up to $45 billion over five years in a technology fund created with Japan's Softbank. In recent months it bought a stake of about 5 percent in U.S. electric car maker Tesla.
Some economists have suggested capital flight from Saudi Arabia -- private citizens parking money abroad because of concern about its safety -- may have increased after a massive government crackdown on corruption around the end of 2017.
However, official data do not show this conclusively, and some commercial bankers believe private capital flows out of the country may actually have decreased because the central bank began monitoring them more carefully after the crackdown.
Sayari stressed that authorities were not imposing capital controls to curb outflows of funds.
"There's no capital flow constraints or restrictions in the kingdom...Investors are of course free to express their views on markets whenever they see opportunities, local or international." (Reporting by Marwa Rashad and Stephen Kalin; Writing by Andrew Torchia; Editing by Emelia Sithole-Matarise)