UPDATE 1-Asia's Iran oil imports drop 22 pct, further cuts likely
(Recasts to focus on Asia-wide imports of Iranian crude; adds quotes, details on imports, sandwich box)
* Top Asian buyers cut Iran oil imports to 961,127 bpd for Jan-June
* Asian buyers cut Iran June imports to 790,054 bpd, lowest since April
* New waiver reviews on U.S. sanctions due Sept and Nov
SINGAPORE, July 31 (Reuters) - Iran's top four oil clients have cut their imports from the Middle Eastern nation by more than a fifth in the first six months of the year, but are soon to face increased pressure from the United States to reduce shipments still further.
The cuts by China, India, Japan and South Korea point to the United States' and European Union's success in reducing Tehran's vital oil cash flows as they try to force Iran to halt a disputed nuclear programme. Oil shipments from Iran are down about 60 percent on average compared to pre-sanction levels.
U.S. lawmakers now want to further toughen the measures that have cost Iran billions of dollars a month in lost revenue, with a goal to squeeze exports to a trickle.
"There is continuing pressure from the United States to reduce Iranian crude imports," said Robin Mills, chief analyst at Manaar Energy Consulting. "They are coming up with a fresh bunch of sanctions to reduce Iranian crude exports further."
The U.S. and EU sanctions have made it difficult for Iran's top clients to insure oil shipments and refineries processing Iranian crude, and forced them to find new ways to pay Tehran because it has no access to international banking networks.
The four Asian countries imported 961,127 barrels per day (bpd) of Iranian crude in the January-June period, down from 1.23 million bpd a year ago, according to official government data and tanker arrival schedules given to Reuters, with the largest percentage cuts coming from India and South Korea.
Japan, the last of the four to report its oil imports for June, imported 185,946 bpd of Iranian crude in the first half of the year, data from the Ministry of Economy, Trade and Industry (METI) showed on Wednesday, down 22.5 percent.
That is less than India's cuts in Iranian oil imports of 43 percent over the first half of the year and South Korea's cuts of 27 percent, but more than China's reduction of about 2 percent from the same six months last year.
"China will be key to the success of the sanctions," Mills said. "They have cut the least and their cuts have been more token. They will be key if the United States wants to cut exports further."
Replacement oil for the lost Iranian barrels have come from Iraq, Oman, United Arab Emirates, Latin America and Africa.
China and India, Iran's top two customers increased their imports from Iraq by 38 percent and 27 percent, respectively, over the first six months of the year compared with a year ago.
India increased its imports from Latin America over the January to June period by 66 percent, raising the region's share of overall Indian imports to 19 percent from 12.6 percent.
Western countries believe Iran's nuclear programme is aimed at making a bomb, while Iran says it is for peaceful purposes.
Iran's president-elect, Hassan Rouhani, who takes office next month, pledged in June to be more transparent on the nuclear programme but no immediate curtailment of its uranium enrichment is expected.
Last month, Washington granted its third 180-day waiver on sanctions applied to Asian countries, including India, China and South Korea, for significantly reducing Iranian oil imports in the six months through May.
Japan won its third six-month waiver in March as part of a different review process. Japan's renewal will come up in September, while the waivers for the other Asian buyers will come up in November-December.
For the month of June, China, India, Japan and South Korea together imported 790,054 bpd of Iranian crude, down from 1.37 million bpd in the same month last year.
That is the lowest for Iran's top four buyers since April, when big drop-offs in barrels shipped into India and Japan cut their total to 635,750 bpd, the smallest in decades.
Japan's import of Iranian crude in June came in at 128,544 bpd, down from 207,800 bpd in June 2012.
Japan's imports may rise in coming months to offset steep cuts made in April due to uncertainties over the continuation of sovereign insurance on tankers carrying Iranian oil.
"So we will recover the loss at some point. Japan's Iran imports are likely to stay mostly steady for the rest of the year as the first six months," JX Nippon Oil & Energy's senior vice president, Akitsugu Takahashi, told Reuters.
JX Nippon is the biggest refiner in Japan, the world's third-largest oil importer.
(Additional reporting by James Topham, Aaron Sheldrick and Osamu Tsukimori in TOKYO; Editing by Tom Hogue)