The land would form part of the proposed Van Long-Kim Bang-Tam Chuc Delacour's Langur Cultural and Ecological Landscape, the provincial People's Committee said.
Local authorities agreed to cooperate on preparing the nomination dossier, describing the proposal as significant for the long-term conservation of natural landscapes and heritage.
Nga, however, said including 3,701 hectares of Phu Tho land could have major implications for land management, development planning, ongoing public and private investment projects, security and defence, as well as local livelihoods.
She called for careful preparation of the dossier, balancing heritage conservation with Phu Tho's long-term development and protecting the interests of the state, residents and businesses.
Phu Tho's Department of Culture, Sports and Tourism was tasked with coordinating with Ninh Binh Province to finalise the dossier for submission to relevant authorities.
The provincial Department of Finance will review projects, investment plans, two industrial clusters and licensed mines within the 3,701-hectare area, while assessing legal risks to avoid disputes and state compensation claims.
The Department of Construction was asked to review construction, urban spatial and technical infrastructure plans covering the area in Lac Thuy and An Nghia communes and surrounding buffer zones.
Local authorities will provide updated data on residents, land use and businesses, while reporting concerns raised by local communities.
Ninh Binh Province held an international workshop in July to assess the outstanding universal value of the proposed Van Long-Kim Bang-Tam Chuc World Heritage site and advance its nomination dossier.
Initial studies have identified significant geological, tropical karst, biodiversity, landscape, archaeological, historical and cultural values in the area.
The landscape is also home to one of the world's rarest endemic primates, the Delacour's langur, making it particularly important for biodiversity conservation.
An estimated 370-400 Delacour's langurs live in the Van Long-Kim Bang-Tam Chuc landscape, representing over 90 per cent of the wild population. About 250-300 are in Van Long Wetland Nature Reserve and 120-150 in the Kim Bang limestone area near Tam Chuc.
Once widespread across northern and north-central Vietnam, the species has declined sharply due to hunting and quarrying. By the early 2000s, only about 40 individuals remained in Van Long.