Heavy calibre mortars and tank fire were also being used in the attacks.
Condemning ethnic Albanian rebels as "murderous thugs", Mr Robertson warned that Macedonia was on the "brink of an abyss".
White House spokesman Ari Fleischer said President George Bush wanted "to make certain that Macedonia is able to take the action they need to combat a problem that has been created by the extremists".
The Macedonian parliament was due to vote on the proposed declaration on Tuesday, but the vote has been postponed. The declaration of a state of war would give the government and security forces more freedom to deal with the rebels.
Villages attacked
On Monday, Macedonian forces continued their bombardment of rebel Albanian positions near the town of Kumanovo.
Helicopter gunships focused their attacks on the village of Slupcane - one of five villages held by ethnic Albanian gunmen.
Heavy calibre mortars and tank fire were also being used in the attacks.
The BBC's Nick Wood said although machine guns could be heard intermittently, there did not appear to be any return fire from the rebels.
Nearly 3,000 ethnic Albanian refugees crossed into Kosovo from Macedonia on Monday in the biggest one-day exodus in five days of fighting, the United Nations refugee agency said.
The refugees said the situation was grave.
"There are many people killed and wounded. There are many people in those villages. They are running out of food," said Sefedin Osmani, a 40-year old man who fled with his wife and five children from a village one kilometre near Slupcane.
Thugs
Nato's Lord Robertson praised the Macedonian Government's firm stance against the guerrillas, and issued his strongest condemnation of them yet.
He called the rebels "a bunch of murderous thugs whose objective is to destroy a democratic Macedonia and who are using civilians as human shields" in a cynical bid to provoke "another Balkan bloodbath".
And President Bush's spokesman said the US President "supports the efforts of the government of Macedonia to fight the extremists who have brought the violence to the region."
But international diplomats are keen to avoid an official declaration of a state of war for fear of alienating more of its ethnic Albanians who make up nearly one-third of the population.
This fear was confirmed on Monday by Arben Xhaferri, head of the Democratic Party of Albanians.
"We are against the tendency to militarise the state. If they proclaim a state of war, we do not want to stay in the coalition," he said.
"Rather than talk about a state of war, we should discuss a state of peace," said the EU's foreign policy chief Javier Solana.
A state of war hands would give enhanced powers to the president and the military. Its provisions would allow for: