But military spokespeople would not be drawn on whether intelligence teams had gathered any new information overnight.
"We do not come out and say what has emerged in this situation, there is operation secrecy on that," Johan Lundgren, a duty spokesperson for the Swedish military told Swedish television network SVT.
The hunt for what media reports suggest may be a damaged Russian vessel is now in its fifth day.
On Monday, The Local confirmed that the military had imposed flight restrictions in the area:
"Civilian flights must not fly under an altitude of 900 metres in the area, for safety reasons," said Marie Tisäter, military spokesperson.
But Tisäter denied early news agency reports that civilian boats had been banned from the area. "There are no such restrictions," she said.
A press conference is expected on Tuesday lunchtime, with Commander Sverker Göranson poised to speak to the media and the government's Defence Committee.
Sweden's armed forces have deployed military vessels and land troops in the Stockholm archipelago, from Möja in the north to Ornö in the south.
By midday on Monday HMS Stockholm had made its way further south to Nåttarö.
It is the biggest operation of its kind in years.
Swedish soldiers were out in force on Monday. Photo: TT
IN PICTURES: 'Foreign activity' in Swedish waters
Tomas Ries, a lecturer at the Swedish National Defence College, said all indicators suggested this was a Russian violation of Swedish waters.
“This fits exactly with their operational patterns when it comes to midget submarines,” he told news agency TT.
Displays of power of this kind are likely to continue for as long as Vladimir Putin remains in charge, he added.
“This is most probably part of a long-term strategy against the West. The basic message is that we need to respect Russia and Russian interests.”
But Russia as well as the Netherlands have both denied that the vessel is theirs.
The search continues in the Stockholm archipelago. Photo: TT
Military spokesman Dag Enander said on Monday:
”We’re using the sensors we have and are searching both from land and water."
He would not comment on whether the armed forces were on the lookout for anyone who might have exited an underwater vessel and made it onto land.
The military will continue its search for at least a couple more days even if nothing turns up in the water.
“We have good stamina,” said Enander.
A witness took this photo, which was released by the Armed Forces. Photo: TT
Sweden’s armed forces on Sunday said it was likely a foreign power was conducting operations in Swedish waters, and have released pictures of a suspicious object seen in the Stockholm archipelago.
In a press conference on Sunday evening said the photographer had “seen something on the surface, and after he took the picture it sunk down again.”
But Grenstad said they had not established that the suspicious object or objects were submarines. Numerous newspaper reports have said that a damaged Russian submarine was the cause of the alert.
"The information that the Armed Forces has does not allow us to draw the conclusion that it is a damaged Russian submarine," he said.
Grenstad said the purpose of Sweden’s current operations was to verify if there is indeed foreign activity in the Baltic Sea off Stockholm. He refused to say whether Russia had been identified as the likely culprit.
He said that three separate sightings had been reported in stretches of water near Stockholm – Kanholmfjärden, Nämdöfjärden and Jungfrufjärden – and that these occurred on Friday and on Sunday.
The search continued on Monday. Photo: TT
Christian Allerman, who twice served as Sweden’s marine attaché in Moscow, said Russia was the likeliest suspect.
”The only nation with a motive is the one that doesn’t want us to continue developing our cooperation with Nato,” he told TT.
Asked what the Swedish military was hunting for in the waters off Stockholm, Allerman said:
”They’re searching either for divers or diving vessels … small submarines or possibly a conventional submarine in the 60 to 70-metre class. The latter is less likely.”
A large number of Swedish military vessels are continuing to search the archipelago.
In other developments on Sunday, Swedish media reported that a Russian research vessel equipped for underwater search operations was heading for Swedish waters. The ship, the Professor Logachev, has a stated destination of Las Palmas, but is currently in the Baltic to the south-west of Hangö in Finland.
The presence of Russian-owned, Liberian-flagged oil tanker NS Concord has also been remarked on in the Swedish media as possibly having a connection to the incident. The tanker was initially heading to Denmark, but has been criss-crossing the seas off Sweden.
But Anders Nordin at the Swedish Coastguard told news agency TT that the NS Concord’s movements were consistent with normal tanker movements.