ISTANBUL—Turkey said at least one soldier was killed in an attack by the outlawed Kurdistan Workers' Party, or PKK, as a recent surge in assaults threatens efforts to end the group's three-decade conflict with the Turkish government.

The assaults by the PKK—which is listed as a terrorist organization by Turkey, the European Union and the U.S.—have targeted Turkish security forces across four provinces in the country's east over the past week, the military General Staff said on Wednesday.

The soldier was killed and another was wounded Tuesday, as the PKK staged an ambush in the province of Van, the military said.

Clashes also erupted between security forces and protesters in the Diyarbakir province. One demonstrator was killed as Turkish authorities removed a Kurdish militant's newly erected statute in the Lice district of Diyarbakir, according to local news reports.

Prime Minister Recep Tayyip Erdogan, who prepares to take over the presidency Aug. 28, has pledged that clinching peace with Turkey's Kurds will remain a priority in his new position. But the PKK strikes threaten to undermine his mandate to end a conflict that has claimed more than 40,000 lives over the past 30 years.

The PKK didn't immediately comment on the clashes. The group has repeatedly warned against the government's push to set up more military outposts in Turkey's heavily Kurdish-populated southeast, amid continuing peace negotiations.

Kurds make up about 18% of Turkey's population of 77 million people.

The attacks come as a PKK-affiliated militia has expanded its operations across Syria and Iraq, fighting the radical Sunni group Islamic State and aiding Iraqi Kurdish forces, the Peshmerga. The PKK-linked militia in Syria has enabled Kurds to establish strongholds in the country across the Turkish border, intensifying the pressure on Ankara to secure lasting peace with Kurds.

The gains have emboldened PKK fighters, who had been largely dormant since the organization's jailed leader, Abdullah Ocalan, declared a cease-fire in March 2013—the most significant step yet in peace negotiations started by Mr. Erdogan in late 2012.

Mr. Erdogan is widely credited among Turks with securing a period of calm, halting the frequent funerals seen in 2011 and 2012 as PKK-military clashes prompted the deadliest year since the 1990s. His sweeping victory in Turkey's first direct presidential elections on Aug. 10 signaled the Turkish leader's peace effort has strong public backing.

In July, the premier's party overcame parliamentary opposition to pass legislation that formalized the peace talks with the PKK, providing legal immunity to officials involved in the negotiations.

Dismissing the recent clashes as provocations, Besir Atalay, the deputy prime minister in charge of the peace process, told the private NTV news channel on Tuesday that the government was working on a road map for the talks.

He suggested that negotiations might include the PKK command in northern Iraq's Qandil mountains—a move that could curb the influence of Mr. Ocalan, who has been jailed since his 1999 capture and long regarded a hero by Kurds in Turkey.

"No one has anything to be concerned about," Mr. Atalay said. "Once we form our road map, everybody's agreement and adherence will be absolutely necessary."

WSJ