By Tetsushi Kajimoto and Minami Funakoshi

TOKYO (Reuters) - Japan's top currency diplomat on Wednesday signaled Tokyo's readiness to intervene in currency markets with the safe-haven yen soaring as Republican Donald Trump closes in on winning the U.S. presidential election.

Asian stocks tanked and the dollar sank more than 3 percent versus the yen on Wednesday as investors faced the real possibility of a shock win by Trump that could upend the global political order.

Masatsugu Asakawa, vice finance minister for international affairs, said he was watching exchange-rate moves with great interest as they were "very rough" and speculative.

"If speculative moves continue, we will take any necessary step," he told reporters after meeting his counterparts at the Financial Services Agency and the Bank of Japan to discuss how to respond to the financial market volatility.

He added that he would continue to watch currency moves closely as London and New York markets open.

Asakawa said earlier on Wednesday that he would consult with Finance Minister Taro Aso on how to respond to the yen's spike, though he declined to comment on whether Tokyo would intervene in the currency market. The finance minister makes the final decision on whether to intervene to stem sharp yen rises.

Earlier on Wednesday, Chief Cabinet Secretary Yoshihide Suga told reporters the government was on guard against excessive market volatility and would act if needed.

Rapid gains in the yen, which is regarded as a safe haven in times of uncertainty, have been a headache for Japanese policymakers concerned about the effect of excessive yen strength on Japan's fragile, export-reliant economy.

Japanese authorities last intervened in currency markets in 2011. Many analysts say it would be difficult for Tokyo to intervene because doing so could breach a Group of 20 agreement to refrain from competitive currency devaluation.

(Additional reporting by Kaori Kaneko, writing by Leika Kihara; Editing by Eric Meijer & Simon Cameron-Moore)