BANGKOK, July 23 (Reuters) - The price of Thai 100 percent B grade white rice RI-THWHB-P1 rose 2.7 percent on Wednesday from last week, and it could rise further after Vietnam announced plans to impose an export tax on rice, Thai exporters said.

The median price quoted by Bangkok exporters was $750 per tonne, up from last week's $730 per tonne but still well below the record of $1,080 per tonne in April. It was the third straight week that prices rose.

"It's a good sign for Thai rice prices," one exporter said. "Vietnam's tax measure should lift not only its prices but also Thai prices."

Vietnam's government said on Tuesday it planned to impose export duties on rice of up to $176 per tonne, which would push its rice export prices up to $800-$1,300 a tonne.

"Thai rice prices could rise another 10 percent next week if the Vietnam tax measure take effect," another Thai trader said.

Exporters said prices were also supported by a Thai government rice-buying programme and steady demand from Nigeria.

Nigeria, one of Thailand's major rice buyers, was believed to be buying up to 100,000 tonnes of parboiled rice per month.

Exporters said Nigerian traders were rushing to buy before the end of October. After that, they will no longer be able to import rice free of duty.