UPDATE 1-China's Sinopec, CNPC skip Iran oil buys for May as U.S. sanctions waivers end -sources

Chen Aizhu and Florence Tan

waivers end -sources@

* Refiners worry over being cut from global finance system-sources

* China is Iran's biggest oil customer

* Unclear how long suspensions will last - sources

* China's crude oil imports from Iran https://tmsnrt.rs/2DYpSXK (Adds details, background, graphic)

SINGAPORE, May 10 (Reuters) - China Petrochemical Corp (Sinopec Group) and China National Petroleum Corp (CNPC), the country's top state-owned refiners, are skipping Iranian oil purchases for loading in May after Washington ended sanction waivers to turn up pressure on Tehran, three people with knowledge of the matter said.

The United States has decided not to renew any exemptions for sanctions on Iran, taking a tougher line than expected on the expiry of the waivers. The waivers were granted last November to buyers of Iranian oil.

China is Iran's largest oil customer with imports at 475,000 barrels a day in the first quarter of this year, according to Chinese customs data.

Two of the sources said Sinopec and CNPC have skipped bookings for cargoes loading in May as companies were worried that taking oil from Iran could invoke U.S. sanctions and cut them out of the global financial system.

A third source said Sinopec, the dominant buyer that makes up majority of China's Iranian oil imports, does not wish to breach a long-term supply contract but has opted to suspend booking new cargoes for now due to the sanction worries.

All of the people with knowledge of the matter requested anonymity due to the sensitive nature of the topic.

Of the five supertankers that loaded Iranian crude in April for China, two have discharged, while another two are waiting off Ningbo and Zhoushan in eastern China to discharge, according to Refinitiv data and its analyst Emma Li. A fifth tanker is heading to Shuidong in southern Guangdong province.

The sources said they didn't know how long the suspensions will last.

Both Sinopec and CNPC declined to comment. The National Iranian Oil Company (NIOC) did not immediately respond to Reuters' email seeking comment.

(Reporting by Chen Aizhu and Florence Tan Editing by Tom Hogue and Kenneth Maxwell)