So far this year, 24,000 migrants have arrived in Italy. In the first three months of the year, arrivals surged by 85% compared with the same period last year. According to the International Organization for Migration, more than 6,000 migrants have arrived in Italy in the past five days, compared with fewer than 200 arriving in Greece.

While arrivals in Italy dropped during the winter, the warm weather has encouraged many more to attempt the passage, with 14,000 arriving in the past six weeks, according to the United Nations High Commissioner for Refugees.

For all of 2014 and the first half of 2015, Italy was the major point of entry for Middle Eastern and African migrants, with most crossing from Libya. More than 300,000 migrants landed in Italy in that period.

Then, last year, a large portion of the flow shifted as Syrians—who made up as much as a quarter of the arrivals in Italy in 2014—began crossing from Turkey to Greece in huge numbers. Virtually no Syrians are currently entering Italy, according to government officials.

Now, the renewed flow into Italy risks creating a large bottleneck of migrants. Under European rules, migrants must file asylum claims in the first EU country into which they set foot. If they travel further, they must be returned to the country in which they original landed.

However, Italy—which had borne the burden of rescuing huge numbers of migrants in the Mediterranean—failed to register the migrants for much of 2014-15, effectively waving them through, with many transiting through Austria in an attempt to reach Germany or other northern European countries.

In recent months, however, Austria sealed its border with Slovenia and Hungary, largely stopping the enormous flows of migrants leaving Greece. That decision, however, left 50,000 migrants stranded in Greece.

Since the start of the year, Austrian officials have threatened to close the Italian border as well, citing Italy’s laxity in registering migrants. Earlier this week, Austria began building a 250-meter-long check point close to the Italian border. On Friday, Austrian President Heinz Fischer played down the threat, telling Italian newspaper La Repubblica that reports of closing the border were “misleading.”

The European Commission has criticized Austria’s plans and warned that any border control measures have to be “exceptional and proportionate.”

Natasha Bertaud, a spokeswoman for the commission, said: “If these plans should materialize, then we’d have to look at them very seriously. The Brenner Pass is essential for the freedom of movement within the European Union. She said tjhat the migration commissioner had spoken on the phone with the Austrian interior minister about the plans, and is awaiting details in writing.

The Italians, following heavy pressure from other EU members, have established four so-called “hot spots,” where they now fingerprint virtually all arrivals, resorting to the use of force in some cases, according to Interior Ministry officials. That means fewer migrants are transiting north to request asylum in countries with more generous migration benefits. Indeed, the number of requests for asylum in Italy rose 50% in March on the year, according to the UNHCR.

Officials worry that the numbers of migrants could soar further this spring and summer. Instability in Libya could drive more Africans who moved to the country to find work to emigrate.

Italy is concerned that a recent deal to deport migrants arriving in Greece could push many to seek an alternative route to northern Europe, possibly by traveling through Albania and making the sea passage to southern Italy. So far, such flows have not materialized, although 5,000 migrants who traveled through the Balkans have entered Italy from Slovenia since the start of the year. Aid workers believe it unlikely that this route will become established because of its difficulty.

The fresh wave of migrants presents particular problems for Italy. Since the migrants are mostly from sub-Saharan Africa—the main nationalities arriving so far this year are Nigerian, Gambian and Senegalese—relatively few will be eligible for refugee status.

Most arrivals nonetheless apply for asylum, as they enjoy at least one year of government support while they wait for an answer from Italy’s long asylum-review process. When they are denied asylum and receive an expulsion notice, they normally remain in Italy.

That raises the “risk of a higher number of illegal immigrants stuck in Italy,” says Mario Morcone, head of migration at Italy’s Interior Ministry. “We’re managing everything for now, but with difficulty.”

Italian officials said on Friday that they plan to ask Brussels for more financial support to deal with the crisis, including agreements to make it easier to forcibly repatriate migrants who are not eligible for asylum.

The EU last year launched a naval operation to gather intelligence on the smuggling rings operating on the Libya-Italy route. On Monday, EU foreign affairs ministers will speak via video-link with the Libyan Prime Minister, Fayez Seraj, to establish how they can expand the scope of the operation and to offer support for setting up an effective coast guard and police force in Libya.

—Valentina Pop contributed to this article.

Write to Manuela Mesco at manuela.mesco@wsj.com