By Saikat Chatterjee

HONG KONG (Reuters) - Asia's policymakers held their fire early on Friday as the "Leave" camp took an unexpected lead in Britain's EU vote, but they said they were ready to steady volatile markets as counting suggested an extremely tight outcome.

Currencies and stocks were the most volatile, with the yen surging to its highest against the dollar in two-and-a-half years, briefly breaking through 100 yen to the dollar. Sterling plunged almost 10 percent to its lowest level against the dollar since 1985. Japanese stocks fell more than 3 percent.

Japan's Deputy Chief Cabinet Secretary Hiroshige Seko told reporters the government was worried about market moves, saying sudden shifts because of the British vote were undesirable

And Japan's top currency official, vice finance minister for international affairs Masatsugu Asakawa, said he would consult with the finance minister on a response to the rough markets.

The yen's strength due to its safe-haven status has been a frustration for Japanese authorities, who want a weaker currency to support exports and the economy. But they have been unable to garner support for intervention to weaken it from other major economies, most notably the United States.

A win for "Leave" could further boost demand for the yen. The 100 per dollar level has been seen as a pivotal point to test the patience of Japan's monetary officials on intervention.

A statement from the G7 group of leading industrial economies was expected on Friday.

The Australian dollar, often sold off in times of heightened market stress, fell heavily against the dollar and the yen.

In Seoul, South Korean Vice Finance Minister Choi Sang-mok said even a close vote to remain in the EU could still have a negative impact on global markets, including South Korea, even though direct trade and finance ties with Britain were small.

"Should Britain choose to stay in the EU by a close vote, it will still have a negative effect on global markets although it will be weaker than the effect Brexit would have," Choi said after a meeting with senior finance officials on the vote.

Choi also said Seoul was ready to steady the market if volatility seemed overdone.

Money market rates were calmer in early trading. Traditional stress markers such as interbank dollar funding rates in Singapore and Hong Kong were broadly steady, while treasury bill and front end government bond yields fell on a flight to safety.

"Because of this matter, we have made preparation in many aspects. We have reserved sufficient liquidity and we are able to handle in different situations," Hong Kong Financial Secretary John Tsang told reporters in the airport on Friday morning before he flew out to Beijing. 

The Hong Kong Monetary Authority has asked banks to maintain ample cash conditions with them. No unscheduled monetary liquidity injection operations have been taken so far, according to an HKMA spokeswoman.

The People's Bank of China injected 170 billion yuan ($25.9 billion) on Friday, taking the net injection for the week to 340 billion yuan, the biggest in two months. But that was seen more related to averting a cash squeeze at the end of the half year next week, as has happened previously, rather than responding to the Brexit vote.

Still, the yuan fell to its weakest level against the U.S. dollar since January 2011.

(Reporting by Saikat Chatterjee; Writing by John Mair; Editing by Martin Howell)