* Nikkei drops 2.9 pct for week, falls for 3 straight weeks
* Foreigners sold net 1.55 trillion of Japanese cash and future stocks in last 2 weeks of June
* Eisai jumps after Alzheimer's drug succeeds in mid-stage trial
* Market cautious against more tariff details from U.S. vs China
By Ayai Tomisawa
TOKYO, July 6 (Reuters) - Japan's Nikkei share average rose for the first time this week in a positive end to Friday, as investors took heart from gains in Chinese stocks and covered their short positions after early wobbles when Washington's tariffs on Chinese imports took effect.
The Nikkei ended 1.1 percent higher to 21,788.14, snapping a four-day losing streak. The index dropped 2.3 percent for the week, posting third weekly declines.
The Nikkei Volatility Index dropped 12 percent to a 1-1/2-week low of 18.16.
According to data released by the Japan Exchange Group, foreign investors sold a net 1.55 trillion yen of Japanese cash stocks and futures during the last two weeks of June. Analysts said given the amount of net selling, it is likely that investors unwound their short positions on Friday as the negative U.S.-China trade news was priced in.
Stocks could experience volatility in the coming weeks, they said, as investors remain jittery about the potential for a full-scale trade war between the world's two largest economies.
The U.S. tariffs on more than 800 goods from China worth $34 billion took effect at 0401 GMT. President Donald Trump has warned the United States may ultimately target over $500 billion worth of Chinese goods, an amount that roughly matches its total imports from China last year.
China's commerce ministry said shortly after the U.S. tariffs took effect that it was forced to retaliate, meaning $34 billion worth of imported U.S. goods including autos and agricultural products are also subject to 25 percent tariffs.
"It's like the first round in a boxing match has just begun. The first punch was given by Trump, and China punched back. There is another 16 billion dollars so Trump will likely give another punch to China soon," said Norihiro Fujito, chief investment strategist at Mitsubishi UFJ Morgan Stanley Securities.
Friday's gainers included tech shares and automakers, with TDK Corp jumping 5.2 percent, Advantest Corp up 3.6 percent, Toyota Motor Corp rising 1.1 percent and Honda Motor Co surging 1.4 percent.
Drugmaker Eisai Co was in the spotlight, jumping 19 percent after it and Biogen Inc said that the final analysis of a mid-stage trial of their Alzheimer's drug showed positive results for patients who received the highest dose.
The broader Topix rose 0.9 percent to 1,691.54. (Reporting by Ayai Tomisawa Editing by Shri Navaratnam)