The United States and its Western European allies today said they were willing to enter into a "realistic dialogue" with the new Soviet leadership but also made it clear that deployment of new American missiles in Europe late next year would take place unless there was a fully agreed upon arms control plan with Moscow first.

The semiannual two-day meeting of North Atlantic Treaty Organization's 16 foreign ministers that opened here today comes at a time of especially great pressures on the alliance.

In part, it is because of the approaching installation of the 572 new U.S.-built Pershing II and cruise missiles, a deployment that has generated considerable controversy in Western Europe. French Foreign Minister Claude Cheysson told reporters today that 1983 would be the most critical year for the alliance since the end of World War II because of the missile deployment and the debate it will bring to a head.

Cheysson, as did other ministers, backed the plan strongly. He said it is "absolutely necessary to move forward with these plans" to keep the pressure on Moscow at the Geneva arms reduction talks or to match Soviet missiles already deployed if no agreements are reached.

The meeting takes place against a backdrop of severe economic problems experienced by all the allies, with sharp disputes among them over trade and protectionist policies and differences over how to trade with the Soviet Union and its allies.

Friday, after the NATO talks, Secretary of State George P. Shultz, four other U.S. Cabinet members and top commissioners of the 10-nation European Community will meet here in an attempt to head off protectionist trade wars and subsidy differences, especially over agriculture, among the allies.

Shultz is on a 13-day trip to seven European countries that started in Bonn Tuesday and will take him to the Netherlands, Italy, France, Spain and Britain.

Today, U.S. officials said that while in Rome, Shultz will meet with Pope John Paul II and Egyptian President Hosni Mubarak, who will also be visiting there. Mubarak comes to Washington next month and officials said the Rome meeting would give them a chance to discuss the situation in Lebanon and President Reagan's peace plan before that visit.

Richard R. Burt, the U.S. assistant secretary of state-designate for European affairs, stressed to reporters that the Reagan zero-option plan for a solution to the problem of European-based missiles remains the only Western-proposed and supported solution.

Under it, the Soviets would dismantle their 333 new SS20 missiles and some older ones while the West would forego deployment of the Pershing and Cruise missiles.

Burt reiterated that while "we are willing to consider any serious Soviet proposals . . . all Soviet proposals to date have failed to meet the basic objective of equality. They have had just one objective: to prevent U.S. deployments" and preserve a Soviet monopoly, he said.

Officials said the deployment must go ahead unless a new Soviet proposal is submitted that is fully negotiated and agreed upon before the December 1983 initial deployment.

U.S. officials said, however, that there was no reason why negotiations could not continue after the deployment begins.