* Fine spring weather leads to rise in new arrivals in past week
* More movement seen as Libya instability worsens
* Italian naval rescue force reinforced (Adds comment from International Organisation for Migration)
By Steve Scherer
ROME, March 21 (Reuters) - Italy's navy has rescued more than 4,000 migrants from overcrowded boats in the Mediterranean Sea south of Sicily in the past four days and several other rescue operations are still going on, officials said on Friday.
This week's warm spring weather has brought calm seas and an surge in new arrivals. Most migrants pay more than $1,000 to criminal gangs in increasingly chaotic Libya to make the crossing to Italy - and the European Union - that killed hundreds last year.
Two suspected people smugglers were taken into police custody when the amphibious assault ship San Giusto and another Italian naval ship arrived in the Sicilian port of Augusta near Syracuse on Friday with more than 1,500 migrants rescued at sea.
While on patrol, the San Giusto picked up one dead refugee and rescued two in critical condition, the vessel's commander Captain Mario Mattesi told Reuters.
"The rescue operations have been reinforced from five to eight ships and all are operating in the area of interest," between Sicily and Libya, Mattesi said.
"The dead man and the two others all showed signs of probable carbon dioxide poisoning and burns from the petrol that was aboard the raft. One of the men was resuscitated on the San Giusto after being rescued," he added.
MORE ARRIVALS FORESEEN
The Geneva-based International Organisation for Migration (IOM) said the arrivals were set to intensify.
"Our feeling and understanding is that there will be more movement because of instability in Libya, more movement of people coming up," spokeswoman Christiane Berthiaume told reporters on Friday.
Italy is a major gateway into Europe for migrants arriving by sea from North Africa and sea arrivals more than tripled in 2013 from the previous year, fuelled by Syria's civil war and strife in the Horn of Africa.
In October, at least 366 Eritreans drowned in a shipwreck near the shore of the Italian island of Lampedusa, which is located about halfway between Sicily and Tunisia. More than 200, mostly Syrians, died in another shipwreck a week later.
However as well as those fleeing strife and civil war, the migrant boats carry many from politically stable but poor countries who cannot find work at home and accept the risks of the voyage in the hope of even low-paid work in Europe.
The IOM says African countries many migrants come from, transit countries including Libya and Tunisia and countries of destination such as Italy had to meet to try to find some solution that could include more temporary permits.
"In a country you need workers to do certain types of jobs, you can have them for a certain period of time," Berthiaume said. "If there were channels for people to migrate, we wouldn't see all these people risking their lives at sea." (Additional reporting by Cristiano Corvino and Stephanie Ulmer-Nebehay in Geneva; Editing by Tom Heneghan)