TEL AVIV — Israel made clear on Thursday that if a new flotilla of pro-Palestinian activists sought to break its naval blockade of Gaza like the one a year ago when its commandos killed nine people, the Israeli military would use force again, including boarding the ships and confronting the activists.

“We will do anything we have to do to prevent a boat from breaking the blockade,” a top naval official said in a briefing for foreign journalists. “If there is the same violence against our forces on board, there is a pretty good chance there will be injuries.” The official spoke on the condition of anonymity under Israeli military rules.

On Israel Radio on Thursday, the military’s chief spokesman, Brig. Gen. Yoav Mordechai, said the army would stop any ship from entering Gaza. General Mordechai added, “There is an unequivocal directive from the government to enforce the naval blockade that is recognized by international law, and we will not allow it to be broken.”

The statements seemed part of a heightened effort to stop another flotilla and to pre-emptively explain Israel’s position if violence ensues.

Groups of Palestinian advocates in chartered vessels are scheduled to depart from a number of European ports this month and assemble into a flotilla heading toward Gaza to challenge Israel’s blockade and commemorate the deaths of a year ago.

Among those expected to participate is an American vessel with several dozen passengers, including the writer Alice Walker and an 86-year-old whose parents died in the Holocaust.

Because of insurance difficulties and political pressure, it remained unclear whether the ship on which last year’s deaths occurred, the Mavi Marmara of Turkey, would join the flotilla as planned. Israel, widely condemned for the commando operation, said that a year ago the ship was dominated by extremists who created the confrontations that resulted in the deaths.

A number of world leaders, including Ban Ki-moon, the United Nations secretary general; Catherine Ashton, the European Union’s foreign policy chief; and Foreign Minister Ahmet Davutoglu of Turkey, have urged the flotilla organizers to drop their plans or wait and see how Gaza fares under changes in Egyptian and Israeli policies.

Four years ago, after Hamas took over in Gaza, Israel and Egypt closed off the territory, preventing most goods and nearly all people from going in and out. Israel began a naval blockade two and a half years ago when it invaded Gaza to stop Palestinian militants from firing rockets into Israel.

But after the commando raid a year ago, international outrage over the deaths, along with the hardships in Gaza, contributed to a shift in Israeli policy. Israel eased its blockade, letting in more goods over land. After the revolution in Egypt this year, Egypt changed its policy toward Gaza, partly reopening its border to people. Today Gaza has plenty of goods available, but its economy remains devastated and unemployment is 40 to 45 percent.

Moreover, Israel continues the naval blockade. The government says its goal is to prevent Hamas from importing weapons by sea. In March, Israel stopped a vessel packed with weapons that it says were Gaza-bound.

This year an Israeli commission concluded that the blockade conformed with international law, as did Israel’s raid on the Mavi Marmara in international waters. The panel included two foreign legal experts who agreed with the conclusions. Turkey dismissed the report as lacking credibility.

Israel’s navy has been training for another flotilla and says it will use a number of tactics before boarding ships and do everything it can to avoid close contact with activists on board.

As it did last time, Israel says it will ask ships carrying aid to Gaza to dock in Israel or Egypt, unload the cargo and allow it to be driven in. Israeli officials say the flotillas’ goal is not to ship aid to the Palestinians, but to challenge and embarrass the Israelis.

The naval officer who briefed foreign journalists said that he did not believe that the coming flotilla would contain arms, but that Israel needed to enforce the blockade indiscriminately to defend against weapons imports by future flotillas. He said searches on board did not work because boats had many areas to conceal things, so the only reasonable way was for the cargo to be unloaded and driven to Gaza.

He said that many of those planning to take part in the flotilla were peace activists, but that they were naïve because “extremists will set the tone” if Israeli commandos board the ships.