Secretary of State John Kerry speaks during a joint press conference at the State Department in Washington on April 30. (JEWEL SAMAD/AFP/GETTY IMAGES)

Secretary of State John F. Kerry is off to a fast start in a job he has long coveted — traveling extensively and tackling nearly every big, thorny foreign policy problem at once. In an around-the-world odyssey last month, he veered from the crisis in Syria to Middle East peace talks to missile-tipped diplomacy with North Korea.

Back in Washington, though, the pace has been glacial. Most senior posts in the State Department remain vacant more than three months into President Obama’s second term; a bottleneck of would-be nominees await approval by the White House.

The contrast between Kerry’s ambitious agenda and the slow changes he has brought to the State Department raises questions about whether he has the political muscle to accomplish what he has started. It also feeds perceptions that the Obama White House has extended control over the department since the departure of Kerry’s predecessor, Hillary Rodham Clinton.

Answering questions from Congress recently, a frustrated Kerry blamed the White House and the slow machinery of Senate confirmation.

“The greatest difficulty I’m finding — now that I’m on the other side of the fence — is, frankly, the vetting process,” Kerry testified last month. “I’ve got some folks that I selected way back in February, when I first came in, and we’re now April, and I’m still waiting for the vetting to move.”

It’s hard to draw firm conclusions about Kerry’s pull with the White House or his likelihood of success with specific policies, but the empty offices at home are a warning flag, several senior department officials said.

Kerry’s travel schedule sets a tone of energy and engagement that is probably welcome abroad, said former U.S. ambassador to Pakistan Cameron P. Munter, but that has to be supported by a smooth bureaucracy at home. “My concern is that to sustain that work of reaching out, he can’t just do all of this himself,” said Munter, a professor of international relations at Pomona College.

Kerry was a frequent troubleshooter during the president’s first term, and he came to the job with extensive foreign policy credentials and the deep respect of many of Obama’s senior aides.

But after nearly 30 years as a senator, Kerry is still getting used to the constraints of speaking for the president, not himself, and of taking orders about what he should not say.

He went off-message several times on his most recent trip, including an unscripted comment revealing his blunt message that Turkish Prime Minister Recep Tayyip Erdogan should not make a long-planned trip to the Israeli-blockaded Gaza Strip. That earned a scolding from Turkish officials, who said Kerry had made it harder for Turkey to pursue political rapprochement with Israel.

Although hiring has slowed and become more cumbersome across the federal government, a trend that spans administrations, the problem is especially acute at the State Department.

As of May 1, no new nominations for non-career ambassadors had been sent to the Senate. That is despite a long list of donors, many of them big-money bundlers, and other Obama political associates said to be in line for plum posts such as London, Paris or Berlin.

Four career Foreign Service officers have been nominated for less-glitzy postings, including Libya and Burkina Faso.

There is no new nominee to replace Thomas R. Nides as deputy secretary for management and resources, and only one assistant secretary has been announced.

Top policy jobs such as the assistant secretaries responsible for Asia and Africa are vacant, and others are occupied by first-term holdovers who had expected to move on to other jobs by now.

The White House has announced only two other nominations for State Department postings: Catherine M. Russell to be ambassador at large for women’s issues and Avril D. Haines to be the legal adviser. Both women held senior jobs in the first-term Obama White House. Russell is the wife of national security adviser Thomas E. Donilon.

“Since I moved from Capitol Hill, I have a new appreciation for how much the confirmation process has become a political football in recent years and what that forces on the vetting process required to announce nominees,” Kerry said in a statement. “The White House and the Administration make the very best out of a tough situation and everyone wants to see good outcomes.”

The empty posts are unusual for a presidential second term.

By May 1, 2005, during President George W. Bush’s second term, four new assistant secretaries, one undersecretary, high-profile nominees (such as the ambassadors to Japan, France and Canada) and a new State Department legal adviser had all been announced.

Obama and Clinton moved faster in their first months on the job in 2009, when the administration had far more open jobs across the government to fill.

By May 1 that year, the White House had announced two deputy secretaries, one undersecretary, nine assistant secretaries, ambassadors to Iraq, Afghanistan, NATO and the United Nations, and at least five additional ambassador-level posts.

The bottleneck has a cascading effect on lower-level jobs, with State Department employees and outside Kerry associates alike waiting for word on new jobs. Employees working on some of the high-profile issues Kerry has embraced do not know who their bosses will be or whether they themselves will stay.

Clinton came to the State Department with the unique power that her political rivalry with Obama had conferred and with a large entourage of political loyalists. She secured wide latitude to hire people of her choosing, without much White House input, and almost immediately began filling the C Street headquarters with former campaign staff and longtime Clinton family associates.

Kerry, as a second-term secretary, has no such deal. He brought in some close aides from his long Senate career, but the very few other high-profile jobs announced are largely going to veterans of the Obama White House.

Kerry’s new spokeswoman, Jen Psaki, worked for Obama in the 2008 campaign and in the first-term White House. She worked for Kerry’s unsuccessful 2004 presidential campaign but has little direct foreign policy experience. Psaki’s new deputy will be Marie Harf, a former CIA analyst and spokeswoman who worked for the Obama 2012 campaign.

In one of his few personnel announcements lately, Kerry on Friday said that former ambassador James Dobbins would be the new special representative for Afghanistan and Pakistan. That post, vacant since January, does not require Senate confirmation.

Julie Tate contributed to this report.