THE United Nations faces a dilemma. While its peacekeeping responsibilities are expanding, its financial resources are shrinking. Member nations are nearly $1 billion behind in their payment of regular contributions to the organization, raising the possibility that the UN won't be able to meet salaries and other obligations.Though this is a crisis, it's hardly a new one. UN Secretary-General Javier Perez de Cuellar has faced funding shortfalls for the last four or five years. Somehow the organization has managed. What makes this year's financial woes at the UN particularly noteworthy is the world body's new activism. A consensus is building that the UN should take on tasks it wouldn't have dreamed of in the past - large-scale intervention in civil strife to prevent the spread of war and to safeguard human rights. The Gulf war, with its UN-sanctioned military action against Iraq and subsequent campaign to eliminate that country's nonconventional weaponry, helped open the door wider to UN undertakings that break the old barriers of sovereignty. The UN has just started a long-term commitment in Cambodia, which could involve thousands of peacekeeping forces and cost $1 billion or more. It may take a substantial role in building peace in El Salvador. The war between Croatian forces and the Yugoslav Army may have reached a stage where UN peacekeepers may intervene. These tasks, plus other ongoing operations in the Middle East, Africa, and elsewhere, are expensive. And they are only one part of the UN's program. Humanitarian and development efforts are costly too. What's needed from members - starting with the United States, which owes hundreds of millions of dollars to the UN - is a firm commitment to pay promptly, without conditions. The placing of conditions on dues payments, in an effort to guide UN policy, has been a habit of US legislators and other officials for years. It's a new era for the UN, with growing responsibilities, a world relatively free of superpower confrontation, and new leadership at the top of the organization next year. Members' old habits of laggard, sometimes grudging payment, must end.