NEW YORK - European stock markets posted modest gains Wednesday as Britain triggered the two-year process to quit the EU, though investors had largely priced in "Brexit" since last year's referendum, dealers said.

US stocks finished mostly higher, with the S&P 500 and Nasdaq up behind a strong performance in energy and some technology shares, although the Dow fell modestly.

The looming split between London and Brussels following last June's shock Brexit referendum has divided Britain and thrown the future of the 28-nation bloc into question.

"We had been waiting nine months, and when it came it was not with a bang but with a whimper," said Manulife Asset Management analyst Will Hamlyn.

'POLITE OPENING'

"The real event to watch is April 4 and April 29, when we get the EU's declaration on Brexit and an EU summit on Brexit," said City Index analyst Kathleen Brooks.

"The polite opening to negotiations has helped suppress volatility and limit market reaction."

European markets barely flinched when the British ambassador to the EU formally handed over the historic letter announcing Britain's intention to leave the bloc.

That step came after Prime Minister Theresa May formally activated Article 50 of the Lisbon Treaty, meaning Britain is set to leave the bloc in 2019.

The pound -- considered a barometer for market concerns over Brexit -- wobbled against the dollar, having enjoyed a brief spike just after May pulled the Brexit trigger.

VOLATILITY AHEAD

"It's business as usual. It's been pretty muted," said David Papier, head of sales at ETX Capital.

"People have known about the trigger of Article 50 for months now, so people have been positioned accordingly and it looks like everything has been priced in. It certainly hasn't shown the volatility that we saw over Brexit (vote) or the US election," he told AFP.

"Once the terms of Brexit, once the costs of Brexit become clear, and once these trade deals have some clarity around them in terms of renegotiation, we might see some more volatility in the market."

Among other markets, oil prices rose solidly after a US petroleum inventory report showed a smaller-than-expected increase in crude supplies and lower stockpiles of gasoline.

KEY FIGURES AT 2030 GMT (4 a.m. in Manila)

New York - Dow: DOWN 0.2 percent at 20,659.32 (close)

New York - S&P 500: UP 0.1 percent at 2,361.13 (close)

New York - Nasdaq: UP 0.4 percent at 5,897.55 (close)

London - FTSE 100: UP 0.4 percent at 7,373.72 (close)

Frankfurt - DAX 30: UP 0.4 percent at 12,203.00 (close)

Paris - CAC 40: UP 0.5 percent at 5,069.04 (close)

EURO STOXX 50: UP 0.3 percent at 3,473.79

Tokyo - Nikkei 225: UP 0.1 percent at 19,217.48 (close)

Hong Kong - Hang Seng: UP 0.2 percent at 24,392.05 (close)

Shanghai - Composite: DOWN 0.4 percent at 3,241.31 (close)

Euro/dollar: DOWN at $1.0767 from $1.0813

Pound/dollar: DOWN at $1.2440 from $1.2457

Dollar/yen: DOWN at 111.04 yen from 111.08 yen

Oil - Brent North Sea: UP $1.09 at $52.42 per barrel

Oil - West Texas Intermediate: UP $1.14 at $49.51 per barrel.

© Agence France-Presse