On Saturday Kuwait said various services were investigating the incident but did not give the magnitude of the spill near Kuwait's southern Ras al-Zour area nor its cause.
On Friday, Kuwait's Environment Public Authority (EPA) said it was taking all appropriate measures in order to protect water outlets from the oil spill.
The Kuwait Petroleum Corporation said teams from Saudi Arabian Chevron and the Oil Spill Response Limited (OSRL) had joined the teams cleaning the coastal waters.
The experts estimated that as many as 35,000 barrels of crude oil may have leaked into the waters off Al Zour, where Kuwait is building a massive US$30 billion (Dh110.2bn) oil complex that includes a 615,000-barrel-per-day refinery.
Kuwaiti media on Sunday quoted local oil experts as saying the spill originated from an old 50-kilometre pipeline from Al Khafji..
Kuwait says an oil spill has struck its waters in the Persian Gulf.
Initial indications suggest the leakage came from an oil tanker.
Saudi Arabia, in a statement published by the official SPA news agency, said it had implemented a "crisis management plan" and was conducting an aerial survey of its oil installations along the coast.
Saudi Arabia and Kuwait jointly operate fields in a shared area known as the Neutral Zone.
Emergency teams have sealed off two power and water desalination plants in the area to prevent contamination.
Bahrain's National Disaster Management Committee has also announced that the kingdom's regional waters are not affected by the oil spill.