Denmark's move to bolster its ban on visiting nuclear-armed warships has focused attention on a NATO ally that already suffers from a reputation as being unreliable.

NATO long has complained that Denmark does not pull its weight in the North Atlantic Treaty Organization. Its annual defense spending 2.2 percent of its gross national product in 1987 is the lowest of NATO's European members except for Greece and Italy.For 31 years, Denmark has had a general policy banning nuclear arms from its soil in peacetime. Until now, Danish officials took the position of assuming that visiting warships would obey.

Last week, Parliament voted to demand that warships be told they cannot carry nuclear arms into Danish ports.