GRAINS-Soybeans at 1-week high on export boost, wheat also firm

* Soybeans rise for third session as strong U.S. exports support

* Corn also firm, weaker dollar lends support

* Wheat slightly higher, market awaits Egypt tender

(Updates with European trading, changes byline/dateline) PARIS/SINGAPORE, May 17 (Reuters) - Chicago soybeans touched a one-week high on Wednesday as export demand continued to support U.S. prices, offsetting the impact of huge harvest supplies in South America. Rain in part of the U.S. Midwest lent some support to soybeans and corn, which was also slightly higher, as it raised the possibility of a slowdown in spring planting. Chicago wheat edged up as rainfall also raised a question mark over the start of the U.S. harvest, while import tenders by Egypt and Tunisia suggested renewed international demand. Further weakness in the dollar also underpinned grains. The most-active soybean contract on the Chicago Board of Trade was up 0.3 percent at $9.79-1/2 a bushel by 1154 GMT, after rising to $9.80-1/2, its highest since May 10. The USDA said on Tuesday that private exporters reported the sale of 132,000 tonnes of soybeans for delivery to unknown destinations during the 2016/17 marketing year. "Despite the high supply from South America usual at this time of year ... the U.S. is continuing to hold its ground in the market fairly well at the moment," Commerzbank analysts said in a note. Traders were also monitoring the weather outlook for the Midwest corn and soybean belt, with showers forecast in parts of the region in the week ahead raising the risk of some planting delays. "Midwest rains still active, slowing late soy seeding in the south," the Commodity Weather Group said in a daily update. CBOT corn added 0.2 percent to $3.68-1/2 a bushel, while wheat was up 0.5 percent at $4.26-1/2 a bushel. Corn remained capped by swift planting progress in the United States last week, and by the weight of global inventories. Wheat traders were awaiting the outcome of a tender being held by Egypt, the world's largest wheat importer. A change to quality specifications, increasing protein requirements, was expected to reinforce the dominance of Black Sea origins like Russian wheat.

But Egypt's return to the market, along with fellow importer Tunisia, could signal an upturn in overseas demand after a lull during the harvest season in North Africa.

Prices at 1154 GMT

Last Change Pct End Ytd Pct Move 2016 Move CBOT wheat 426.50 2.25 0.53 408.00 4.53 CBOT corn 368.50 0.75 0.20 352.00 4.69 CBOT soy 979.50 3.25 0.33 1004.00 -2.44 Paris MAT wheat Sep 166.75 0.50 0.30 168.00 -0.74 Paris maize Jun 170.00 0.50 0.29 166.00 2.41 Paris rape Aug 366.50 1.50 0.41 408.50 -10.28 WTI crude oil 48.89 0.23 0.47 53.72 -8.99 Euro/dlr 1.11 0.00 0.23

Most active contracts - Wheat, corn and soy US cents/bushel, Paris futures in euros per tonne

(Reporting by Naveen Thukral in Singapore and Gus Trompiz in Paris; Editing by Richard Pullin and Susan Fenton)