Three powerful oil-producing nations--Saudi Arabia, Venezuela and Mexico--met in London and reiterated plans to boost production later this year but did not say how much or when. The three nations account for about a fifth of the world's oil output and are the top suppliers to the United States, which has been calling on its allies in the Persian Gulf states and elsewhere to increase production to stabilize prices. Venezuela's minister of energy and mines, Ali Rodriguez, said that the three countries have agreed to do that, but "the problem is, we must agree how much to increase and exactly when it will happen." A year ago, the Organization of Petroleum Exporting Countries and other major oil producers agreed to cut production following an agreement by the same three nations. The result has been a strong recovery in oil prices from near $10 a barrel to more than $30 a barrel. Light sweet crude for April delivery closed Thursday at $31.63 a barrel, down 8 cents, on the New York Mercantile Exchange.