ROME — More than 20,000 migrants have reached Italy in the last week, a sharp spike that has left the Italian government considering whether to deny landing rights to independent rescue ships not flying the Italian flag if it does not get more help from the European Union.

The number of migrants risking the perilous crossing of the Mediterranean from Libya often increases in warmer months, but this week’s surge is extraordinary even compared with the already high summer numbers of recent years.

The spike in migration has inflamed one of the most divisive debates in Italian politics, and worsened tensions between Italy and the European Union. And the role of rescue ships operated by humanitarian groups and nongovernmental organizations has now moved to the center of that debate.

Right-wing parties, which celebrated victories in Sunday’s municipal elections, have latched onto the climbing number of asylum seekers as a vote-getter. Some have argued that the center-left government is incapable of stanching the flow of migrants, while others accuse the government of having a secret plan to swell the number of immigrants with the intention of one day granting them voting rights.