Iran exported oil at levels higher than allowed under Western sanctions for a fourth straight month in February, ship loading data seen by Reuters showed, raising the risk of a crackdown if Washington feels economic pressure is being relaxed too quickly.The higher sales to Iran's main clients, mostly in Asia and including Turkey, have come after a temporary agreement eased some sanctions aimed at undermining Tehran's nuclear program.Under the deal, Iran's exports are supposed to be held at an average 1 million barrels per day (bpd) for the six months to July 20. But shipments to Asia have topped that level at least since November, according to customs and ship tracking data. As talks on a final nuclear deal resumed in Vienna on Tuesday, US President Barack Obama was again facing pressure from US lawmakers to be tougher on Iran.US senators have written to Obama to say Tehran should not be allowed to circumvent sanctions as world powers work towards a lasting agreement with the Middle East country."Iran cannot be allowed to be open for business," 83 senators wrote in a letter. "We view this period as one fraught with the danger of companies and countries looking to improve their commercial position in Tehran."February crude loadings by Iran's top four buyers - China, India, Japan and South Korea - rose to 1.16 million bpd versus 994,669 bpd in January, according to the loading plan.Adding in oil lifted by Turkey - which came in at 105,824 bpd in January and 117,857 bpd in February - Tehran's exports have busted the sanctions limits at least since November. The loading volumes exclude condensate, a light oil, that Iran exports to China and others.Since sanctions were imposed in 2012 and more than halved Iran's oil exports, China, India, South Korea, Japan and Turkey have bought nearly all Iranian crude exports.The Obama administration believes Iran's exports will fall in coming months and exports through late July will average 1 million bpd. To ensure that happens, Washington could put more pressure on Iranian crude buyers to slash purchases.Indian government sources have said refiners there must cut their Iranian oil imports by nearly two-thirds from the first quarter after the United States urged New Delhi to hold the shipments at end-2013 levels.The temporary deal between Iran and world powers, struck in November and implemented on Jan. 20, also freed up $4.2 billion in back oil payments to Tehran in return for curbs on its nuclear program.BREACHING CEILINGThe intake of Iranian oil by Asian buyers alone has topped 960,000 bpd since November, government and industry data has shown, and adding in an average 100,000 bpd of crude for Turkey, exports have breached 1 million bpd at least since then. With loadings for the first two months of the year - for February and March arrivals - also holding above 1 million bpd, according to the loading document, exports look set to breach the cap for the first quarter of the year, allowing up to three weeks for shipments to China, Japan and South Korea.China lifted 502,500 bpd in February, again taking its purchases back to pre-sanctions levels. The nation's refiners received 564,536 bpd in January.In comparison, China imported 428,840 bpd of Iranian oil for all of 2013, according to customs data. China's February import numbers are not due out until later this week.China's total oil imports from Iran may rise in 2014 as state-run trader Zhuhai Zhenrong Corp is negotiating a new condensate contract, Reuters has reported.India lifted 304,286 bpd of crude in February, according to the loading data. Iran's second-biggest client imported 412,000 bpd in January and averaged 195,600 bpd in arrivals in 2013.South Korea loaded 214,286 bpd in February, mostly for March arrival dates. February arrivals from Iran, meanwhile, doubled from a year earlier as refiners hiked purchases ahead of maintenance shutdowns starting from March.Japan loaded 140,000 bpd in February, according to the loading program. It purchased 210,517 bpd from Iran in January, after reducing imports by 6 percent in 2013, marking its lowest daily crude imports from Iran in more than 30 years.Japan's February import numbers are due later this month.CONDENSATE EXPORTSIran's exports to Asia, including condensate, were 1.25 million bpd in February, up 62,000 bpd from the previous month, the loading document showed. That was on par with a 13-month high in oil and condensate arrivals in Asia in January.Including Turkey, Iran's crude and condensates exports rose to 1.37 million bpd in February, up 74,000 bpd from January.