LONDON — One of the prime reasons for the wave of migrants, refugees and asylum seekers washing into Europe is the deterioration of the conditions that Syrians face in Lebanon, Jordan and Turkey, a worsening largely caused by sharp falls in international funding from United Nations countries, officials and analysts say.

That shortfall in funding, in contrast with the greater resources provided by Europe, is prompting some to make the hazardous journey who might otherwise remain where they are. The United Nations Syria Regional Refugee and Resilience Plan, which groups a number of humanitarian agencies and covers development aid for the countries bordering Syria, had by the end of August received just 37 percent of the $4.5 billion appeal for needed funds this year.

António Guterres, the high commissioner for refugees, recently said that his agency’s budget this year would be 10 percent smaller than in 2014, and that it could not keep up with the drastic increase in need from the long Syrian conflict, which includes shelter, water, sanitation, food, medical assistance and education. The United Nations refugee agency’s funding for Syria this year is only at 43 percent of budgeted requirements.

The World Food Program, another United Nations agency that is a cornerstone of refugee aid, has also been making painful cuts. It is more than 63 percent underfunded for 2015, the agency said, and recently had to halve the monthly stipend to 211,000 Syrians in regional refugee camps.