The White House blamed Senate Republicans for putting US national security at risk by thwarting the swift confirmation of a long list of President Barack Obama's ambassador picks -- some to hot-spot nations.
National Security Advisor Susan Rice said 48 of Obama's nominations for ambassador overseas were pending and awaiting confirmation by the chamber.
"I am focused every day on keeping our country secure and our citizens -- at home and abroad -- safe," said Rice in a White House blog post.
"These goals are hindered and our stature abroad is weakened when we are not fielding a full team," she said.
Rice said a quarter of US ambassadors in Africa were awaiting Senate confirmation, even as extremist threats, like the Boko Haram Islamist group, pose rising national security risks.
She also warned that the US envoy's seat to the Association of Southeast Asian Nations (ASEAN) also remained vacant, at a time when regional tensions are soaring over China's territorial claims in the South China Sea.
In addition, US positions at the World Bank, International Monetary Fund, the Inter-American Development Bank and the European Bank for Reconstruction and Development remain unfilled.
- 'Unacceptable' -
White House spokesman Josh Earnest also raised concerns about the jammed-up ambassadorial nominations in his daily briefing.
"These nominees have been waiting an average of 262 days, and these delays are simply unacceptable," he said.
"It's time Republicans in the Senate ended their obstruction and allowed these qualified individuals to do their important work protecting American interests around the world."
Democratic Senate Foreign Relations Committee Chairman Robert Menendez complained about the situation last week, also putting the blame squarely on Republicans.
"They have consciously chosen a strategy to do nothing, pass nothing, approve nothing and leave key diplomatic posts unfilled for months -- threatening national security and our ability to conduct foreign policy," Menendez said.
Menendez's committee sent the nominations of a clutch of ambassadors to the Senate floor on Tuesday, though it was unclear when they would get a full vote.
Unless Republicans and Democrats in the chamber can agree on a deal on blanket confirmations, it could take many months before all the ambassadors are in place if each one is considered separately.
The nominees passed out of the Foreign Affairs committee Tuesday included envoy picks for Egypt, Qatar and Iraq -- crucial posts given the wave of violence and unrest that is sweeping the Middle East.
The delay in confirming ambassadors has been put down to a general logjam in the Senate, where partisan wrangling between Democrats and Republicans in a mid-term election year has become extreme.
The flow of ambassadorial nominations has been crawling along amid a row over a decision by the Democratic majority last year to eliminate a 60-vote threshold for some judicial nominee votes -- a move minority Republicans opposed.
The Senate has confirmed only five ambassadors to foreign nations since the beginning of May: to Switzerland, Belize, Angola, Jordan and Peru.
Republicans argue that Democratic Senate Majority Leader Harry Reid, an Obama ally, is in charge of scheduling votes on nominees, and has in several cases given priority to judges over US envoys abroad.
Republicans have also pilloried Obama's choices for several ambassadorial posts after widely lampooned hearing performances by nominees earlier this year, including several who did not know basic facts about nations where they were to be stationed.
And Republicans sources said the administration had yet to nominate an envoy to Romania or to complete paperwork for new envoys to France, Ireland and Slovenia.
Presidents have long made a practice of awarding plumb ambassadorial posts in places like London and Paris to top campaign fundraisers, while more testing posts are reserved for seasoned career State Department officials.